Excellent Cold-Workability Exhibiting High-Strength Steel Wire or Steel Bar or High-Strength Shaped Article, and Process for Producing Them

ABSTRACT

There are provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, or high-strength shaped article and a process for producing them. In particular, there is provided a process comprising carrying out hot working at 350 to 800° C. of a steel ingot, cast slab, steel slab or steel semifinished product having a C content of not greater than the solid solution limit of carbon of ferrite phase at Ael point and not greater than 0.010 mass % and being free of any cementite, or having a C content of &gt;0.01 to 0.45 mass % to thereby obtain a material whose average crystal grain diameter in a cross section perpendicular to the longitudinal direction is ≦3 μm, and thereafter carrying out cold working of the material to thereby attain formation of a ferrite structure whose average crystal grain diameter in a cross section perpendicular to the longitudinal direction is ≦500 nm.

TECHNICAL FIELD

The present invention relates to an excellent cold-workability exhibiting high-strength steel wire or steel bar or a high-strength shaped article such as a screw and a bolt utilizing those characteristics, and a process for producing those steel wire or steel bar and high-strength shaped article.

BACKGROUND ART

Conventionally, regarding a screw or a bolt produced by shaping a steel wire or a steel bar by cold working such as cold heading, and rolling and/or machiening, and other high-strength parts for machine structural use, a steel wire rod produced by hot processing is processed into a steel wire having the desired wire diameter by cold working, the steel wire obtained is subjected to a so-called spheroidizing annealing treatment that makes a cementite in a metal structure spherical, by heating the steel wire at a temperature of about 700° C. for a long period of time of from about ten-odd hours to on day and night to soften the material, thereby improving cold-workability such as cold heading, and the material is shape-worked into a product shape for various uses. However, because the shaped article thus processed does not meet strength necessary as a final product by the above softening treatment, it is required that the shaped article is subjected to thermal refining such as quenching/tempering.

Further, thereafter it is general that the shaped product is appropriately subjected to a surface treatment or the like and shipped as a product. Thus, in the production steps of the conventional high-strength parts for machine structural use, because of a previous softening treatment to a raw material and thermal refining to a shaped article after cold working, it requires long time, and at the same time, is complicated, loss of heat energy is large and productivity is low. Thus, there were problems in the points of increase in heat treatment cost and administration of delivery time.

As a measure that can solve such a problem, a process is proposed, that produces an excellent cold-workability exhibiting steel for cold heading without carrying out spheroidizing annealing to a steel wire rod generally carried out, in order to improve cold headability of the steel wire rod produced by hot processing (for example, Patent Document 1). This process is that by forming C in a steel as a carbide other than Fe₃C at a temperature higher than a cementite formation temperature, solid solution C content in the steel is substantially reduced to suppress formation of cementite and therefore perlite, that inhibit deformation resistance and deformability, and on the other hand, the amount of ferrite first precipitated is greatly increased, thereby greatly improving the cold workability.

However, according to this process, the spheroidizing annealing treatment can be omitted, but tensile strength of the steel wire obtained reaches only up to 500 MPa. Therefore, in the case that high strength as a shaped article obtained by cold heading is required, thermal refining such as quenching/tempering becomes necessary. Further, the problem remains such that in order to form C in the steel as a carbide other than Fe₃C, V which is relatively expensive metallic element is added, and this incurs increase in costs.

Further, a process is proposed, that is not required to apply thermal refining such as quenching/tempering after formation of a product form by carrying out the shaping including cold heading (for example, Patent Document 2). In this process, of steel wire rods conventionally produced, a material in which a metal structure has quenching/tempering structure, the product of yield strength and work hardening coefficient is satisfied with a specific condition range, and cracks do not occur in a predetermined compression test is selected as a raw material used. However, in this process, it is not necessary to apply the spheroidizing annealing treatment requiring a long period of time, to a steel wire as a raw material for cold heading into a hexagon head bolt and the like, but it is necessary to apply the quenching/tempering treatment to the steel wire before carrying out cold heading.

Under such a circumstance, the present inventors solve all of the above problems, develop the technology that can omit the thermal refining treatment that is carried out after cold working, as well as the conventional softening treatment such as the spheroidizing annealing treatment that is carried out before cold working, and propose this technology as a novel invention (Patent Document 3).

In this invention, because a required and defined strain is introduced into a steel piece or a steel product of C: less than 0.45 mass % at a rolling temperature in a range of from 350 to 800° C., warm caliber rolling is carried out.

By this, a steel comprising, as a main phase, a ferrite structure having an average grain size in a cross section perpendicular to a rolling direction of 1 to 2 μm or less can be produced, and it is possible to produce an excellent cold-headability exhibiting steel having a reduction of area of 70% or more and a tensile strength of 800 MPa or more as mechanical properties, without applying quenching, or quenching/tempering treatment. When this steel is used, a shaped article such as a screw or a bolt, having excellent strength can be produced by cold working including cold heading. Based on this invention technology, the present inventors of this application have advanced the investigations on a measure for securing excellent characteristics and effect possessed by the steel obtained by this technology, and for further improving strength while holding high level of cold workability. In doing so, regarding characteristics of mechanical properties of a steel to be produced, it was set to exceed the level of 600 MPa or more (desirably 800 MPa or more) of the tensile strength TS targeted in the proposed invention (Patent Document 3) of the target value, desirably to greatly exceed those values, and to maintain the level of 65% or more (desirably 70% or more) of the reduction of area RA targeted in the above patent application as possible, desirably to exceed those. Specifically, it was targeted to obtain a steel wire or a steel bar, provided with:

Case 1: TS≧700 MPa, and RA≧65%, where further desirably, regarding RA, to increase 70% or more;

Case 2: TS≧1000 MPa, and RA≧70%;

Case 3: TS≧1500 MPa, and RA≧60%.

Thus, where the steel wire or the steel bar is one provided with the characteristics that the tensile strength TS is high level and a strength-ductility balance used as a substitute for the tensile strength TS and the reduction of area RA is high level, shaping by cold heading becomes easy even to the production of parts conventionally shape-worked by mainly machiening, such as pivot class, in addition to fixing parts such as a screw and bolt, and dramatic improvement in shape-working yield of from a steel wire or a steel bar to a high strength pivot class (the conventional level is generally low as about 60 to 65%) becomes possible.

In the course of investigations by the inventors, a prospect was obtained that a steel wire or a steel bar having further high strength than the conventional one and excellent cold workability, and a high-strength shaped article could possibly be obtained by using a steel of a component system having a chemical component composition substantially free of a cementite as a raw material, applying the technology of the above-described proposed invention to this, using this as a raw material (a steel wire rod), and applying an appropriate cold working to this. However, to make this actually possible, it is necessary to melt-produce a steel such that a cementite is not substantially formed in a standard structure of a steel, as a chemical component composition. For example, a refining step for producing a high purity pure iron for an electromagnetic steel plate, or a steel having C content lower than that, is required. To achieve this, even in the case of using either of a converter and an electric furnace as a finery in a steelmaking step, a decarburization reaction is further promoted to a molten steel tapped from those furnaces by a vacuum refining in an appropriate vacuum finery, thereby refining into an ultralow carbon steel, and in addition to this, in a casting step such as continuous casting, a countermeasure for ensuring cleanliness of a steel by reoxidation prevention of a molten steel is desired.

[Patent Document 1] JP-A-2000-273580

[Patent Document 2] JP-A-2003-113422

[Patent Document 1] Japanese Patent Application No. 2003-435980

DISCLOSURE OF THE INVNETION

The invention of this application is to solve the above problems, and first, there is provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized by having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 500 nm or less and being free of a cementite.

Second, there is provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized by having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ael point, and having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 500 nm or less.

Third, there is provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized by having a C content of 0.010 mass % or less, and having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 500 nm or less.

Fourth, there is provided a high-strength shaped article, characterized by having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 500 nm or less and being free of a cementite.

Fifth, there is provided a high-strength shaped article, characterized by having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ael point, and having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 500 nm or less.

Sixth, there is provided a high-strength shaped article, characterized by having a C content of 0.010 mass % or less, and having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 500 nm or less.

Seventh, there is provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized by having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 500 nm or less, and having mechanical properties that a tensile strength is 700 MPa or more and a reduction of area is 65% or more.

Eighth, there is provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized by having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 500 nm or less, and having mechanical properties that a tensile strength is 1500 MPa or more and a reduction of area is 60% or more.

Ninth, there is provided an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized by having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 500 nm or less, and having a hardness of 285 or more in terms of Vickers hardness HV.

Tenth, there is provided a high-strength shaped article, characterized by having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 500 nm or less, and having a hardness in at least one cross section of cross sections in optional directions of 285 or more in terms of Vickers hardness HV.

Eleventh, there is provided a high-strength shaped article, characterized by having a C content of from more than 0.01 to 0.45 mass %, having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 500 nm or less, and having a tensile strength TS of 900 MPa or more.

Twelfth, there is provided a coiled steel wire rod or steel wire, characterized in that regarding a region of 90% or more of an area on a C cross section of a material to be rolled, an average crystal grain size is finely grained into 1.0 μm or less.

Thirteenth, there is provided a process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized in that a steel ingot, cast slab, steel slab or steel semifinished product having a cementite-free ferrite structure is subjected to warm working to prepare a material having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 3 μm or less, and the material is then subjected to cold working to form a ferrite structure having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 500 nm or less.

Fourteenth, there is provided a process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized in that a steel ingot, cast slab, steel slab or steel semifinished product having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ael point is subjected to warm working to prepare a material having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 3 μm or less, and the material is then subjected to cold working to form a ferrite structure having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 500 nm or less.

Fifteenth, there is provided a process for producing a high-strength shaped article, characterized by being produced using the excellent cold-workability exhibiting high-strength steel wire or steel bar produced by the above production processes 13 and 14 by cold heading, cold forging and/or machiening.

Sixteenth, there is provided a process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar, characterized in that a steel ingot, cast slab, steel slab or steel semifinished product having a C content of from more than 0.01 to 0.45 mass % is subjected to warm working to prepare a material having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 3 μm or less, and the material is then subjected to cold working to form a ferrite main phase structure having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 500 nm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph exemplifying the relationship between a rolling condition parameter Z and a ferrite average grain size.

FIG. 2 is a view showing groove dimension sites of diamond style, square style, and oval style caliber rolls.

FIG. 3 is an example (the case of Example 3) of a ferrite structure photograph by SEM of a cross section in L direction of a steel (a steel wire rod) after warm working in the intermediate step of producing the steel according to the invention of this application.

FIG. 4 is a graph showing a rising state of a tensile strength TS with increasing cold working rate when the cold working rate is expressed by converting into an industrial strain e, and further showing the difference between the Examples and the Comparative Examples at that time.

FIG. 5 is a graph showing a drop state of a reduction of area RA with increasing cold working rate when the cold working rate is expressed by converting into an industrial strain e, and further showing the difference between the Examples and the Comparative Examples at that time.

FIG. 6 is a graph comparing quantification of the level value of a tensile strength TS and a reduction of area RA, and a balance state of the tensile strength TS and the reduction of area RA, regarding the Examples and the Comparative Examples.

FIG. 7 is a graph comparing the level of a tensile strength TS to a C content in a steel wire between the Examples and the Comparative Examples.

FIG. 8 is a graph comparing the level of a reduction of area RA to a C content in a steel wire between the Examples and the Comparative Examples.

FIG. 9 is a photograph showing a state that a torsional delay fracture specimen of M1.6 pan-head machine screw was set.

FIG. 10 is a graph showing a rising state of a tensile strength TS with increasing cold working rate when the cold working rate is expressed by converting into an industrial strain e, and further showing the difference between the Examples and the Comparative Examples at that time.

FIG. 11 is a graph showing a drop state of a reduction of area RA with increasing cold working rate when the cold working rate is expressed by converting into an industrial strain e, and further showing the difference between the Examples and the Comparative Examples at that time.

FIG. 12 is a graph comparing quantification of the level value of a tensile strength TS and a reduction of area RA, and a balance state of the tensile strength TS and the reduction of area RA, regarding the Examples and the Comparative Examples.

FIG. 13 is an example (the case of Example 2) of a ferrite structure photograph by TEM of a cross section in L direction of a steel (a steel wire) after cold working obtained by the production process according to the invention of this application.

FIG. 14 is a graph comparing the level of a tensile strength TS to a C content in a steel wire between the Examples and the Comparative Examples.

FIG. 15 is a graph comparing the level of a reduction of area RA to a C content in a steel wire between the Examples and the Comparative Examples.

BEST MODE FOR CARRYING OUT THE INVNEIN

The invention of this application has the constitution as described above, and the characteristics related thereto.

Consequently, next the illustrative embodiment of the invention of this application, and the limitation reasons in the embodiment of this illustrative embodiment are described.

[I] A carbon steel to a low alloy steel wherein a main phase of a metal crystal structure is a ferrite and a C content is from more than 0.01 to 0.45 mass %, and a carbon steel to a low alloy steel wherein a main phase of a metal crystal structure is substantially cementite-free, a C content is not greater than a solid solution limit of carbon in a ferrite phase at Ael point, and a C content is 0.010 mass % or less.

(1) Provision of Chemical Component Composition and Crystal Structure

The first characteristic of a chemical component composition of the excellent cold-workability exhibiting high-strength steel wire or steel bar, and high-strength shaped article according to the invention of this application is a carbon steel to a low alloy steel wherein a main phase of a metal crystal structure is a ferrite and a C content is from more than 0.01 to 0.45 mass %, and the second characteristic of a chemical component composition of the excellent cold-workability exhibiting high-strength steel wire or steel bar, and high-strength shaped article according to the invention of this application is that the chemical component composition of the excellent cold-workability exhibiting high-strength steel wire or steel bar, and high-strength shaped article is applied over the carbon steel to the low alloy steel wherein a main phase of a metal crystal structure is substantially cementite-free, a C content is not greater than a solid solution limit of carbon in a ferrite phase at Ael point, or a C content is 0.010 mass % or less. Here, in the component design, what C content is determined is appropriately made so as to satisfy the mechanical properties desired in the objective uses to be produced, by referring to the relationship between a C content and a tensile strength TS described in the specification of the invention of this application (for example, FIG. 7, FIG. 8), in the case of giving other component element content.

In the above, beyond the lower limit 0.01 mass % of the C content may be considered beyond the solid solution limit of carbon in a ferrite phase at Ael point.

This is because even in the case that a metallic element, such as Cr or Mo, capable of forming Fe(3-X)MXC by replacing a part of Fe element of Fe3C with this element M is contained, such a solid solution limit of C content is similar to a solid solution limit of carbon in a ferrite phase at Ael point in a component system of a carbon steel if it is about a content of an alloy element contained in the steel wire or steel bar comprising a low alloy steel.

The solid solution limit of carbon in a ferrite phase at Ael point can be estimated using, for example, the known calculation software “Thermo-calc” (the “Thermo-calc” is calculation in an equilibrium state, but because cooling condition in the actual production is not an equilibrium state, it does not say that the solid solution limit can completely be estimated). Further, it is required that a metal structure comprises a ferrite as a main phase. It is one of structural elements in the production process of the high-strength steel wire or steel bar of the invention of this application. According to the above proposed invention by the inventors, the crystal structure of a steel wire rod having an average grain size of 3 μm or less prepared by warm rolling should be a steel comprises a ferrite as a main phase.

On the other hand, in the provision of the above chemical component composition, it is not necessary in the invention of this application to depend on addition of an alloy element in order to improve strength of a material. Therefore, it is not necessary to positively carry out addition of an element that promotes to improve quenching property, such as Cr or Mo, and other homologous elements; and Cu or Ni which is a solid solution reinforcing element and other homologous elements. In addition, it is desirable that the above alloy element is not added from reduction in production cost.

Further, it is more desirable to limit an Si content to 1.0 mass % or less and an Mn content to 2.0 mass % or less in order to further surely suppress formation of a cementite in a material and not to incur increase of the production cost due to addition of a large amount of the alloy element. Regarding the provision of chemical component composition in the invention of this application as above, in either of a steel wire or steel bar, a shaped article represented by a screw or a bolt, and steel ingot and steel slab, Al or the like as a deoxidizing agent, a valuable metal such as Ti, Nb or V as a dispersion precipitation reinforcing element, and P, S, N and the like generally handled as harmful impurities, that are component elements other than C, Si, Mn, Cr, Ni and the like, are not limited in their contents. However, regarding the deoxidized element, the content of the essential level on the conventional refining and casting technologies should be ensured. Regarding elements generally handled as impurities, it should be limited to the unavoidably contaminated content, and it should not particularly be limited to an ultra-low content. Regarding other valuable elements, the content is not particularly limited, but it is not necessary to contain the same. By this, the invention of this application is capable of sufficiently solving the problems.

In particular, in the invention of this application, it is the important characteristic that the chemical component composition may be one that does not generate martensitic transformation by quenching treatment. The reason for this is that where the structural elements according to the production process of the invention of this application are satisfied, the targeted tensile strength of 800 MPa or more, desirably 900 MPa or more, more desirably 1200 MPa or more, and further desirably 1500 MPa, is obtained, and further, a steel maintaining a reduction of area RA in high level according to those tensile strengths is obtained.

Thus, the fact that mechanical characteristics excellent in balance between high strength and high ductility greatly depends on the point that a hard cementite which is a factor to deteriorate cold workability is not substantially formed.

In the steel wire or steel bar, or the shaped article according to the invention of this application, the judgment as to whether it is substantially cementite-free is not always easy as the practical problem. Accordingly, it can be estimated by a qualitative analytical value of the practical C content in the routine operation. From this, in the invention of this application, it is defined that the C content is not greater than the solid solution limit of carbon in a ferrite phase at Ael point, from the metallographic judgment. Further, in a component system of a general low alloy steel, it is defined to be 0.010 mass % or less as a range of the C content that is considered to not form a cementite.

In the above, because of being not greater than the solid solution C concentration (mass %) in a ferrite phase at Ael point, it is actually a cementite-free structure. In either of a carbon steel and a low alloy steel, the C concentration (mass %) that a cement-free structure is actually obtained can be estimated using, for example, the known calculation software “Thermo-calc” (the “Thermo-calc” is calculation in an equilibrium state, but because cooling condition in the actual production is not an equilibrium state, it does not say that the solid solution limit can completely be estimated). Thus, in the invention of this application, it is possible to design a material having the above-described high strength, and further having excellent cold workability (a high-strength steel excellent in balance between strength and workability) in a steel having a cementite-free ferrite structure. Conventionally, an example that an excellent cold-workability exhibiting high-strength steel wire or steel bar is realized by such a component design is not found.

On the other hand, it is more desirable to limit the Si content to 1.0 mass % or less and the Mn content to 2.0 mass % or less in order to further surely suppress formation of a cementite in a material and not to incur increase of the production cost due to addition of a large amount of the alloy element.

The invention of this application attaches importance to be a cementite-free steel as a basic principle for obtaining high-strength characteristic as described above. Accordingly, in defining the chemical component composition as above, it is not necessary to depend on addition of the alloy element. For this reason, it is not necessary to positively add an element that promotes to improve quenching property, such as Cr or Mo, and other homologous elements; and Cu or Ni that is a solid solution reinforcing element, and other homologous elements. In addition, it is desirable that the above alloy element is not added from the point of reduction in production cost. Therefore, it is desirable that the above each element does not exceed the content that is unavoidably contaminated in refining and melting steps of a steel.

Further, although not particularly defined in the invention of this application, there is no need to add Ti or Nb that is an element useful for precipitation reinforcement, and other alloy elements. By the cementite-free component system of the invention of this application, sufficient tensile strength can be secured, and this is useful for the reduction in production cost.

As described above, the C content of the steel (a steel wire or steel bar, and a shaped article) according to the invention of the application is basically designed so as to be cementite-free. Therefore, the standard structure of the steel is always a ferrite structure.

Regarding the provision of chemical component composition as above, in either of a steel wire or steel bar, a shaped article represented by a screw or a bolt, a steel ingot, a steel slab and the like, Al or the like as a deoxidizing agent, a valuable metal such as Ti, Nb or V as a dispersion precipitation reinforcing element, and P, S, N and the like generally handled as harmful impurities, that are component elements other than C, Si, Mn, Cr, Ni and the like, are not limited in their contents. However, regarding the deoxidized element, the content of the essential level on the conventional refining and casting technologies should be ensured. Regarding elements generally handled as impurities, it should be limited to the unavoidably contaminated content, and it should not particularly be limited to an ultra-low content. Regarding other valuable elements, the content is not particularly limited, but it is not necessary to contain the same. By this, the invention of this application is capable of sufficiently solving the problems.

(2) Provisions of Average Grain Size of Ferrite, Tensile strength TS and Reduction of Area RA

In either of the steel wire or steel bar, and the shaped article represented by a screw and a bolt according to the invention of this application, an average grain size of a ferrite in the invention of this application is defined. Specifically, on a cross section (C directional cross section) in a direction perpendicular to their longitudinal directions, it is defined to be 500 nm or less. The reason for defining the average grain size of a ferrite like this is to secure strength of the steel wire or steel bar, and the shaped article in the desired level or more. That is, the definition is for obtaining excellent characteristics that, in the steel wire or steel bar, the tensile strength TS is at least 700 MPa, and according to the uses, the tensile strength TS is 1000 MPa or more, and further desirably 1500 MPa or more, and further for obtaining a steel having excellent balance of both wherein reduction of area RA is also maintained in high level according to each level of the tensile strength TS, for the purpose of securing ductility. The balance between the tensile strength TS and the reduction of area RA means the previously described balance as shown below:

Case 1: TS≧700 MPa, and RA≧65%, further desirably, by further improving the level of the reduction of area RA, TS≧700 MPa, and RA≧70%;

Case 2: TS≧1000 MPa, and RA≧70%;

Case 3: TS≧1500 MPa, and RA≧60%.

By the combination of each level of the tensile strength TS and the reduction of area RA as above, the steel wire or steel bar can be supplied to desired purposes according to the uses.

The reason for this definition is for making it possible to improve working pass yield, and supply a shaped article of quality level that has not conventionally been realized, in working the shaped article. Further, to products, such as pivot class, that are conventionally produced from a steel wire or steel bar by machiening, the steel wire or steel bar having high strength and excellent ductility of the invention of this application is appropriately supplied according to the uses, and as a result, its working yield is remarkably improved.

Further, where the average grain size of the ferrite is reduced 200 nm or less, the combination of the above tensile strength TS and reduction of area RA of the steel according to the invention of this application can easily and stably be obtained with further high level, and this is desirable. In the shaped article represented by a screw and a bolt, it can be considered that the average grain size in at least one cross section of cross sections in optional directions is substantially equal to the average grain size in C directional cross section in the steel wire or steel bar.

According to the process for producing an excellent cold-workability exhibiting high-strength steel according to the invention of this application, it became possible to design a material (a high-strength steel having excellent balance between strength and workability) having the above-described high strength in a low carbon steel to an ultra-low carbon steel, any realized example having not conventionally been found out. Based on such a material design, the possibility of novel development of a high-strength steel having further excellent balance between strength and workability is expected.

(3) Provision of Hardness

In the steel wire or steel bar according to the invention of this application, hardness is defined as the strength characteristic in place of the tensile strength TS. It is desirable that the hardness is 285 or more in terms of Vickers hardness HV. When the Vickers hardness HV is 285 or more, the tensile strength of about 900 MPa is secured. On the other hand, in the shaped article represented by a screw or a bolt according to the invention of this application, preparation of a tensile test specimen may not be easy depending on its shape. Therefore, the provision by hardness should sufficiently be made as the mechanical characteristic in place of tensile strength. From such a standpoint, to the shaped article represented by a screw or a bolt, the provision by hardness as a substitute for tensile strength further takes the importance as the characteristic level evaluation for practical products. Regarding the shaped article, further desirably the Vickers hardness HV is 300 or more corresponding to about 1000 MPa of tensile strength TS.

Practical embodiment of the process for producing the steel wire or steel bar, and the shaped article according to the invention of this application having the above-described characteristics, and the limitation reason thereof are described below.

(4) Basic Constitution of Production Process According to the Invention of this Application (Provision of Combined Step of Warm Working+Cold Working)

The basic characteristic of the production process according to the invention of this application is that as a process for producing a raw material used to produce the excellent cold-workability exhibiting steel wire or steel bar according to the invention of this application, warm working is applied to a predetermined material under appropriate conditions, and by this warm working, fine-grained structure steel is prepared. It is desirable that a crystal grain size of the material obtained is small as possible. Specifically, it is necessary that an average grain size in a cross section (C directional cross section) perpendicular to a longitudinal direction of the material obtained by warm working is 3 μm or less. Next, cold working is applied to the material under appropriate conditions. By this cold working, a fine-grained structure steel in which crystal particles in the cross section (C directional cross section) in a direction perpendicular to a longitudinal direction of the material after cold working are further finely grained is obtained. The fine-grained structure obtained here is that the main phase is ferrite, and cold working is applied. Therefore, it generally shows a form of a so-called bamboo structure stretched in a cold working direction.

Thus, an excellent cold-workability exhibiting high-strength steel is obtained. In this case, in the cold working, when the fine-grained structure steel prepared by the above warm working is used as a raw material, it has been found that even though material strength remarkably increases, reduction of workability is extremely small, which is extremely advantageous. This novel finding that has conventionally been difficult to anticipate constitutes the foundation of the invention of this application. Thus, the reason that an appropriate cold working described below is applied to the material in which fine crystal particles are already formed just before applying the cold working is that the following great advantages are obtained. It is not necessary to apply a spheroidizing annealing treatment to the steel obtained, after shape-working, and it is also not necessary to apply the thermal refining by quenching and tempering to the shaped article obtained, even after shape-working.

(5) Warm Working Conditions (Provisions of Working Temperature, Plastic Strain and Reduction)

As the practical embodiment of the production step of the above excellent cold-workability exhibiting high-strength steel, the desirable warm working condition to a predetermined steel ingot, cast slab or from steel slab to steel rod is that the working temperature should be in a range of from 350 to 800° C. Further, in such a case, plastic strain introduced into a material and remained therein should be secured. The amount of this plastic strain can be obtained with calculation by the known three-dimensional finite element method (the value is expressed “E”), and it is desirable that E is 0.7 or more. The reason for employing such a warm working condition is to finely grain crystal particles as a method of realizing high strength of a steel without substantially utilizing reinforcement mechanism by phase transformation. The inventors find in the invention as the above-described Patent Document 3 that making the reduction of area RA of a steel be a predetermined level or more by doing so is extremely useful to make the cold workability such as cold headability excellent. In the above warm working condition, strain of a material that can relatively be easily obtained on operation (in the invention specification of this application, called “industrial strain”, and expressed “e”) can practically be used in place of “F” as a measure. The industrial strain e is a function of a total reduction R of a material, and is represented by the following equation (3):

e=−ln(1−R/100)  (3)

wherein R is a total reduction represented by the following equation (1):

R={(SO−S)/SO}×100  (1)

wherein R: total reduction (%) applied to cast slab or steel slab

SO: C directional cross section area of cast slab or steel slab just before initiation of warm working

S: C directional cross section area of material obtained after completion of warm working

When the value of R corresponding to ε≧0.7 is calculated using the above equation (3) and equation (1), R≧50% is obtained. Therefore, in the warm working, the total reduction of a material R≧50% may be employed in place of the plastic strain ε≧0.7. On the other hand, the inventors of this application focus attention on that the average grain size of ultra-fine particles formed by the warm strong working (large plastic working by one warm pass) depends on the working temperature and strain rate, introduce the following equation (4):

Z=log[(ε/t)exp{Q/(8.31(T+273))}]  (4)

wherein ε: average plastic strain

t: time (s) from initiation of rolling to completion thereof

Q: constant (when crystal structure is bcc, 254000 J/mol)

T: rolling temperature (° C.); in the case of multi-pass rolling, Zener-Hollomon parameter represented by temperature averaging rolling temperature of each pass

as the rolling condition parameter, and find that the crystal grain size is fine-grained with increasing the rolling condition parameter Z. The relationship between the rolling condition parameter Z and the average ferrite grain size is illustrated in FIG. 1. That is, FIG. 1 shows that the fine-grained structure having the average ferrite grain size of 1 μm or less is obtained by controlling the rolling so as to be Z≧11. Therefore, it become possible to make the average ferrite grain size of a material be less than 3 μm by controlling the warm rolling temperature so as to satisfy Z≧11. Further, as the warm working method, either of warm rolling and warm forging may be employed. In such a case, the plastic strain in a material is contemplated to be homogenized by working in plural directions by multi-passes (in the case of the warm forging, plural forging schedule), and this is desirable.

(6) Cold Working Condition (Provisions of Working Temperature, Plastic Strain and Reduction)

It is desirable that the cold working condition to be previously applied to a high-strength and excellent workability exhibiting material having a fine-grained structure prepared by the above warm working is that the cold working temperature is less than 350° C. Where the temperature reaches a temperature higher than this temperature due to the generation of heat by working during cold working, the degree of rise of tensile strength is lowered, which is not desirable. Next, it is necessary to secure the residual strain introduced into a material by cold working according to the desired tensile strength. From such a standpoint, it is desirable that the cold working is applied such that the plastic strain E obtained by a three-dimensional finite element method is at least 0.05 or more. By this, the cold worked structure of a crystal shows a form stretched in the working direction, the grain size in C directional cross section to the working direction is fine-grained, and the rise of tensile strength is secured. In such a case, the decrease amount of the reduction of area RA is suppressed small. In the above cold working condition, by transmitting e as the “industrial strain” explained by the above equation (3) in place of the measure of E as the working amount, when the total reduction R of a material corresponding to ε≧0.05, R≧5% is obtained. Therefore, in the cold working, the total reduction of a material R≧5% may be employed in place of the above plastic strain ε≧0.05.

In the above, either of the known cold wire drawing method and cold rolling method may be employed as the cold working method. In the cold rolling method, it is desirable to be conducted by the known combined roll method. Where the form of a steel produced by the cold working is a steel wire or steel bar, it can be applied to the use of a shaped article in which high strength and good cold workability are particularly required in carbon steel wires for cold heading according to JIS G 3539, and further the use of a product in which high strength and good cold workability are particularly required in steel species of relatively low C content region among hard steel wires according to JIS G 3505.

[II] Examples<Carbon Steel to Low Alloy Steel, in which Main Phase of Metal Crystal Structure is Ferrite, and C Content is in a Wide Range of from More Than 0.01 Mass % to 0.45 Mass %>

Example 1 and Example 2 partially differ in the production steps of the high strength steel wire or steel bar according to the invention of this application, and Examples 1 and 2 and Example 3 differ in the chemical component composition, in addition of the production steps. Therefore, the test method and test result are separately described in Examples 1 and 2 and Example 3.

EXAMPLE 1 AND EXAMPLE 2 [II] (1)-1) Common Test Between Example 1 and Example 2 (Warm Rolling Step and Characterization Test of Test Material Obtained)

Example 1 and Example 2 were tested as follows. A steel having the chemical component composition shown in Table 1 was melted using a vacuum melting furnace, and cast into a steel ingot. This chemical component composition is, for example, that, in the chemical component composition defined by SWRCH5A belonging to the carbon steel wire rod for cold heading according to JIS G 3507, having Si content of 0.10 mass % or less, Si contains 0.30 mass % exceeding the Si content. However, there is the characteristic in the point that C content is low 0.0245 mass %.

TABLE 1 Chemical component composition (mass %) Supplied C Si Mn P S N sol. Al Example 1 0.0245 0.30 0.20 0.010 0.001 0.0018 0.032 Example 2

The steel ingot obtained above was shaped into a steel bar of 80 mm square by hot forging. Metal structure of those steel bars was ferrite main phase, and the average grain size of the ferrite in C directional cross section was about 20 μm or less. A material for rolling was collected from the above each steel bar of 80 mm square, shaped into 18 mm square by warm multi-pass caliber rolling in multi-directions, and ice-cooled to prepare a steel bar. This warm rolling is to prepare a raw material for the steel wire or steel bar according to the invention of this application, and was carried out under the condition that the average crystal grain size in a cross section perpendicular to the longitudinal direction of the material obtained by this warm rolling is 3 μm or less.

The warm caliber rolling method in which the average crystal grain size is 3 μm or less as described above was carried out under the following conditions. The rolling material of 80 mm square shaped by the above warm forging was heated to 550° C., subjected to warm rolling of 19 passes, wherein the reduction in each one pass is about 17%, by a diamond style caliber roll (see FIG. 2, the upper figure) as shown in Table 2 at a rolling temperature in a range of from 450 to 530° C., and shaped into 24 mm square. Next, the material was subjected to warm rolling with an oval style caliber roll having the maximum short axis length of 11 mm and a long axis length of 52 mm (a and b in FIG. 2, the lower figure, respectively, provided for R=64 mm), and finally subjected to one pass warm rolling with a square style caliber roll, thereby shaping into 8 mm square by total 21 passes. The total reduction of from the warm rolling material (80 mm square) to the 18 mm square material was 95%. The outline of the pass schedule is shown in Table 2.

TABLE 2 Pass No. Caliber No. Caliber style 1 1 Diamond 2 3 2 4 5 3 6 7 4 8 9 5 10 11 6 12 7 13 8 14 9 15 10 16 11 17 12 18 13 19 20 14 Oval 21 15 Diamond

In the above one pass warm rolling by the above oval style caliber roll, the 24 mm square bar is rolled with the above oval style caliber roll. Therefore, the proportion of the maximum short axis length 11 mm in C directional cross section of the material after rolling to the opposite side length 24 mm in C directional cross section of the material before rolling is considerably small as (11 mm/24 mm)×100=46%, and the reduction calculated from the hole dimension at this time is considerably large as 38%. Therefore, the one pass warm rolling with the oval style caliber roll constitutes the condition to further promote fine-graining of the ferrite grain size in the 18 mm square steel bar after completion of the warm rolling. In the rolling process with the diamond style caliber roll up to the 19^(th) pass, the rolling that continuously passes every two passes through the same caliber roll (a so-called “double pass”) is appropriately carried out in order to approach the cross section shape of the material to the square as possible, and each double pass was counted as two passes, respectively. Further, the material was rotated around the lengthwise direction axis core every pass of rolling to change the rolling direction, and multi-pass rolling in multi-directions was carried out. Furthermore, by virtue of the working exothermic heat, heat radiation amount is relatively small in the relatively low temperature region even in the rolling temperature region of the warm working. Next, the 18 mm square steel bar prepared by the above warm rolling method was reduced in diameter by machiening to process into a 6.0 mm diameter steel wire rod.

The reason that the diameter is reduced from 18 mm square to 6.0 mm diameter is as described below. In this Example, M1.6 pan-head machine screw (a diameter in the effective cross section in a screw portion is 1.27 mm) defined in JIS Bu1111 as the use of a steel wire was selected. Therefore, it is intended to prepare a material by which 1.3 mm diameter is obtained by the cold wiredrawing working with the wiredrawing rate 95% as the target or the cold rolling working with the total reduction 95% as the target. The reason for selecting M1.6 pan-head machine screw is as follows. To form a cross-shape recess (depressed portion giving torque with a screw driver) on the top of the screw by heading, an extremely excellent cold headability is required. Therefore, it is for the evaluation as to whether having remarkably excellent cold headability, by a cross-shape “recess shaping test” of M1.6 pan-head machine screw described hereinafter.

In the above, the grain size in C directional cross section of the 18 mm square steel bar prepared by warm rolling was equalized over the entire surface.

This 6.0 mm diameter test material for characterization was collected, and the tests of the following items were carried out. The steel wire rod processed into 6.0 mm diameter after collecting this test material for characterization was continuously subjected to the tests of Example 1 and Example 2.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test: This test has the object to obtain basic data as to whether it is a material having high level balance between strength and cold workability, such that strength is particularly excellent, and simultaneously cold workability is also considerably excellent.

2) Harness measurement test by Vickers hardness testing machine: This test is effective to confirm correlativity with tensile strength, as one of strength characteristics, and in the case that collection of a tensile test specimen is difficult. This test was conducted based on the method defined in JIS Z 2244.

3) Measurement test of ferrite grain size (d) by microscopic test: An appropriate microsection is prepared from each test material, and as an average grain size of a ferrite constituting the main phase by a microstructure of a metal crystal, an average ferrite grain size in a cross section (C directional cross section) in a direction perpendicular to the longitudinal direction of the test material (consistent with the longitudinal direction of the above 18 mm square steel bar) is measured. In such a case, actually the average ferrite grain size in C directional cross section was obtained by observing the microstructure in L directional cross section.

The above test results relating to the warm rolling material are shown in Table 3.

TABLE 3 Average Wire ferrite diameter grain size of test Tensile Reduction Vickers in C cross C Production material strength of area hardness section Supplied (mass %) step (mm) TS (Mpa) RA (%) Hv (−) d (μm) Example 1 0.0245 Warm rolling 6.0 702 78.6 255 0.7 Example 2

The following facts are understood from the test results in Table 3. Despite that the steel wire rod by this warm rolling is a low carbon steel having C content of 0.0245 mass %, any special reinforcing element is not added, and only warm rolling is performed, the tensile strength TS maintains high strength of 702 MPa, and at the same time, the characteristic that the reduction of area is an extremely high level of 78.6% is obtained. Thus, it is apparent to be a material having excellent balance between strength and shapability. This is due to that a fine-grained structure steel that the microstructure of metal crystal comprises a ferrite as a main phase and the ferrite grain size is 0.7 μm is obtained by the condition within the range of the invention of this application. Thus, even in the low carbon steel having C content of 0.0245 mass % that has not ever seen as a steel wire rod for cold heading generally practically used, tensile strength attains high level of 700 MPa or more, and further the reduction of area secures extremely high level.

On the other hand, using a steel wire rod of 6.0 mm diameter after collecting the above 6.0 mm diameter test material for characterization, test was conducted to produce a steel wire by cold working into 1.3 mm diameter from 6.0 mm diameter by cold wiredrawing in Example 1 and by cold rolling in Example 2.

[II] (1)-2) Different Test Between Example 1 and Example 2 (Cold Working Step and Characterization Test of Test Material Obtained) [II] (1)-2)-a) [Cold Wiredrawing Method in Example 1 and Characterization Test of steel wire obtained]

The above 6.0 mm diameter steel wire rod at ordinary temperature (a steel wire rod obtained by working into 18 mm diameter, and then machiening into 6.0 mm diameter, as described above) was successively wire-drawn by wiredrawing dies of die No. 1 to No. 17 as shown in Table 4 to produce a steel wire of 1.3 mm diameter. Material temperature during wiredrawing was less than 200° C.

TABLE 4 Cold wiredrawing in Examples 1 to 6 (6.0 mm diameter→ 1.3 mm diameter) Die No. 1-11 12-13 14-17 Steel wire diameter after 5.6*⁾ to 2.1 1.9 to 1.8 1.6 to 1.3 wiredrawing (mm) Note *⁾Starting diameter of material for wiredrawing is 6.0 mm diameter.

In the overall wiredrawing steps in Example 1, wiredrawing could easily be conducted from 6.0 mm diameter to 1.3 mm diameter without applying spheroidizing annealing and other softening treatment. A test material for characterization in a form of a drawn wire was collected from the steel wire of 1.3 mm diameter (total reduction of drawn wire: 95.3%). The characterization test method is as follows, and 1), 2) and 3) are the same as described above.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test

2) Harness measurement test by Vickers hardness testing machine

3) Measurement test of ferrite grain size (d) by microscopic test

4) Recess formation test of machine screw: A steel wire of wire diameter 1.3 mm was preliminarily shaped by header working in the production step of M1.6 pan-head machine screw defined in JIS B1111, and the predetermined cross-shape recess (depressed portion such as cross-shape for screwing the screw by a screw driver) is formed on the top by cold heading. It is a test of observing the state that cracks generate on this recess at the time of shaping, with a magnifying glass of 10 magnifications. In general, generation state of recess crack greatly varies depending on the recess shape of a machine screw, but a cross-shaped recess formation on M1.6 pan-head machine screw is extremely severe head-shaping. In the present specification, it was positioned that this test is a practical test, and at the same time, particularly excellent evaluation test for cold headability. The state that crack was not observed was indicated “Good”, the state that fine cracks were observed, but it was good as a whole was indicated “Slightly good”, the state that cracks were observed was indicated “Crack”, and the state that great cracks occurred was indicated “Great crack”.

5) Torsional torque test of machine screw: A screw portion is formed by cold rolling on a screw intermediate having the above-described recess head-shaped from a steel wire of wire diameter 1.3 mm to prepare M1.6 pan-head machine screw. Torque is increased in this until breakage of the screw by an appropriate torque measurement device according to the method defined in 5.4 “Torsional test” of JIS B 1060 “Mechanical properties and performance of meter type thread rolling screw having carburizing, quenching and tempering applied thereto”. The torque value required to induce breakage (breaking torque (kgf·cm)) was measured. The object of this test is to evaluate “Torsional strength” that is one of characteristics of mechanical properties to a fixing part such as a screw or a bolt. Hereinafter the same in the present specification. In the case of M1.6 pan-head machine screw, it is desirable that the breaking torque is 3.0 kgf·cm or more.

The test results of Example 1 are shown in Table 5.

TABLE 5 Wire Total Average ferrite diameter reduction of grain size in C Recess Torsional of testing cold Tensile Reduction Vickers directional shapability breaking C material wiredrawing strength of area hardness cross section of machine torque Test (mass %) (mm) (%) Strain TS (Mpa) RA (%) Hv (-) d (nm) screw (kgf · cm) Example 1 0.0245 1.3 95.3 3.06 1567 — 355 182 Crack 3.38

The following facts are understood from the test results in Table 5. That is, the steel wire of 1.3 mm diameter obtained in Example 1 is a low carbon steel having C content of 0.0245 mass %, any specific reinforcing element is not added, and any heat treatment such as quenching and tempering or any softening treatment is not applied. However, its tensile strength TS is remarkably high as 1567 MPa, and the reduction of area RS is considerably high level as 60.2%. The reason for this is as follows. As shown in Table 3, the raw material is that due to the warm rolling, the tensile strength TS is already extremely high as 702 MPa, and the Vickers hardness Hv is also in an extremely high level of 355, and is a fine ferrite structure steel (the average ferrite grain size in C directional cross section is 0.7 μm) that the reduction of area RA is 78.6%, thus already reaching high level. To such a material, cold working by the total reduction of 95.3% is applied by wiredrawing.

Thus, the reason that although the steel wire of Example 1 is a low carbon steel, high strength and high ductility are imparted to the steel wire after cold working is due to that the crystal particles of the steel wire are constituted of a fine ferrite main phase. Specifically, the 1.3 mm diameter steel wire of Example 1 has the ferrite main phase having the average ferrite grain size in C directional cross section of 182 nm and showing the form extended to the direction of cold wiredrawing working in a bamboo structure shape.

From the conception that the ferrite grain size in C directional cross section after the cold working is controlled by the working strain amount, the investigation is made here from the measurement value of the grain size before and after the cold working. In the case of Example 1, the average ferrite grain size in C directional cross section in the steel wire rod (a steel wire rod just before initiation of the cold working) prepared by warm rolling was 0.7 μm (see Table 3). On the other hand, when the ferrite grain size in C directional cross section of the steel wire rod (wire diameter: 6.0 mm) obtained by warm working is indicated d1, and the total cross section reduction by cold wiredrawing to the steel wire rod is indicated R (%), an average ferrite grain size d2 in C directional cross section of the steel wire (wire diameter: 1.3 mm) after cold wiredrawing is estimated by the following equation (5):

D2=(1−R/100)½×d1  (5)

Because R is 95.3%, and d1 is 0.7 μm, it is calculated as d2=152 nm. This calculated value 152 nm well consists with the measured value 182 nm.

Therefore, in the process for producing a steel wire or steel bar according to the invention of this application, it is effective to use the above equation (5) as the means for controlling the ferrite grain size in C directional cross section of a steel wire rod in producing the steel wire by cold working of a steel wire rod of a cold rolling material.

Next, the steel wire thus produced according to the invention of this application was that when the reduction of area RA was 60.2% as a measure of ductility level, cracks generated to the recess shaping that is the shaping process at which extremely severe cold heading is applied, like M1.6 pan-head machine screw, in the state without thermal refining such as quenching and tempering. However, when the torsional torque test was performed, 3.38 kgf·cm satisfying 3.0 kgf·cm or more which is the desirable breaking torque value as M1.6 pan-head machine screw was obtained, and it was understood to have high torsional strength.

[II] (1)-2)-(b) [Cold Rolling Method in Example 2 and Characterization Test of Steel Wire Obtained]

Test for producing a steel wire was conducted by rolling the 6.0 mm diameter steel wire rod at ordinary temperature (a steel wire rod obtained by shaping into 18 mm diameter by warm rolling, and then machiening into 6.0 mm diameter, as described before) into 1.3 mm diameter by cold rolling with each combined roll in the first step to the third step, as shown in Table 6.

TABLE 6 Cold rolling in Example 2 Rolling step No. Second First step step Third step Number of pass 8 passes 10 passes 5 passes Steel wire diameter after rolling 5.7*⁾ to 3.3 3.1 to 1.8 1.6 to 1.3 (mm) Wire diameter of test material 3.3 1.8 1.3 (mm) Rolling total reduction in collection 69.8 91.3 95.3 stage of test material (%) Industrial strain in collection stage 1.20 2.41 3.06 of test material e (%) Note *⁾Start diameter of rolling material is 6.0 mm.

That is, a steel wire was produced by rolling from 6.0 mm diameter to 3.3 mm diameter by 8 passes in the first step, rolling from 3.3 mm diameter to 1.8 mm diameter by 10 passes in the second step, and rolling from 1.8 mm diameter to 1.3 mm diameter by 5 passes in the third step. The material temperature during rolling was less than 200° C. In all of those rolling steps, cold rolling could easily be performed from 6.0 mm diameter up to 1.3 mm diameter without applying spheroidizing annealing and other softening treatment. During this, as a test material for characterization, the test materials for characterization after each cold rolling were collected in three stages of 3.3 mm diameter (total reduction: 69.8%), 1.8 mm diameter (total reduction: 91.0%) and 1.3 mm diameter (total reduction: 95.3%). The characterization test methods are as follows as described before.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test

2) Measurement test of hardness by Vickers hardness testing machine

3) Measurement method of ferrite grain size (d) by microscopic test

4) Recess formation test of machine screw

5) Torsional torque test of machine screw

The above test results are shown in Table 7.

TABLE 7 Average grain Wire Total size of ferrite diameter reduction in C Recess Torsional of test of cold Tensile Reduction Vickers directional shapability breaking C material rolling Strain strength of area hardness cross section of machine torque Test (mass %) (mm) (%) e TS (Mpa) RA (%) (Hv (−) d (nm) screw (kg × cm) Example 2 0.0245 3.3 69.8 1.20  922 — — — — — 1.8 91.0 2.41 1147 — — — — — 1.3 95.3 3.06 — — 328 — Crack 2.92

From the above test results, the following is understood. Meanwhile, the point that the production condition of Example 2 differs from that of Example 1 is that the working was conducted by cold rolling in place of cold wiredrawing. Other conditions are the same. In Example 2, the wire diameter of the test material collected differs from that of Example 1, but the tensile strength TS is high level as 922 MPa in wire diameter 3.3 mm (total reduction: 69.8%), and 1147 MPa in wire diameter 1.8 mm (total reduction: 91.0%). Further, the Vickers hardness HV reaches extremely high level as 328 in wire diameter 1.3 mm (total reduction: 95.3%).

Comparing Example 2 with Example 1 in the Vickers hardness at wire diameter 1.3, Example 2 (cold rolling) is 328, and Example 1 (cold wiredrawing) is 355. Therefore, it is understood that when other conditions are the same, increase in hardness is slightly large in the case by cold rolling rather than cold wiredrawing. Thus, it is understood that even though the cold working method to a raw material (a steel wire rod) is either of cold wiredrawing and cold rolling, the similar high-strength steel wire is obtained if the raw material has a ferrite main phase structure having the same chemical component composition of the raw material (a steel wire rod) just before cold rolling, and the same crystal structure state, particularly the same average ferrite grain size in C directional cross section, and the tensile strength TS and the reduction of area RA are the same. Further, even though only cold rolling is applied without applying spheroidizing annealing, the torsional breaking torque of M1.6 pan-head machine screw is 2.92 kgf·cm, and high torsional strength near the desirable level of 3.0 kgf·cm is exhibited.

EXAMPLE 3

As Example 3 within the scope of the invention of this application, the following test was conducted. A commercially available steel wire rod of 13 mm diameter belonging to SWRCH5A of the carbon steel wire rods for cold heading defined in JIS G 3507, having the chemical component composition shown in Table 8, and produced by hot rolling was used. The component of this steel wire rod has carbon C of 0.03 mass %, and is similar to the component composition of the steels used in Example 1 and Example 2. However, Si content of the steel used in Example 3 is 0.03 mass %, differing from Si=0.30 mass % in Examples 1 and 2, and satisfies the Si content provision (Si≦0.10 mass %) of SWRCH5A.

TABLE 8 JIS Chemical component composition (mass %) corresponding sol. Supplied to component C Si Mn P S Al Example 3 SWRCH5A 0.03 0.03 0.20 0.016 0.010 0.035

The above hot rolled steel wire of 13 mm diameter was worked into a steel wire rod of 6.0 mm diameter by multidirection and multipass warm rolling with caliber rolls at a rolling temperature in a range of from 450 to 530° C. The warm rolling method conducted a caliber roll rolling in which a diamond style, a square style and an oval style were appropriately combined, according to the preparation process of the steel wire rod for supplying to Example 1 and Example 2. A test material for characterization was collected from the steel wire rod of 6 mm diameter thus obtained by warm rolling, and the tests of following items were conducted. The 6.0 mm diameter steel wire rod after collection of the test material for characterization was continuously supplied to the test (as described before) of Example 3.

1) Measurement Test of Tensile Strength (TS) and Reduction of Area (RA) by Tensile Test 2) Measurement Test of Ferrite Grain Size (d) by Microscopic Test

The above test results are shown in Table 9.

TABLE 9 Wire Average diameter ferrite grain of testing Tensile Reduction size in C C Production material strength of area cross section Supplied (mass %) step (mm) TS (Mpa) RA (%) d (μm) Example 3 0.03 Warm 6.0 817 75.0 0.8 rolling

The following is understood from the test results of Table 9. The microstructure of metal crystal of the steel wire rod of Example 3 comprises a ferrite as a main phase, and the ferrite grain size is that the average ferrite grain size in C directional cross section is fine particle of 0.8 μm as shown in the microstructure photograph in L directional cross section by SEM (scanning electron microscope) of FIG. 3. Therefore, it is understood that despite of a low carbon steel having C content of 0.03 mass %, high level characteristics that the tensile strength TS secures high strength of 817 MPa, and simultaneously the reduction of area RA is 75.0% are obtained, and it is a raw material having excellent balance between strength and shapability. This is due to that it is a material satisfying the preparation condition (production condition) of a raw material (a steel wire rod) for producing the excellent cold-workability exhibiting steel wire or steel bar of the invention of this application, and produced by warm rolling.

Next, in Example 3, the test of producing a steel wire by the following cold rolling was carried out using the 6.0 mm diameter steel wire rod prepared by warm working as described above, as a raw material. The cold rolling method was that a steel wire was produced by cold working up to 1.3 mm diameter according to the first step to the third step of the cold rolling in Example 2 shown in Table 6. During this, steel wire test materials having been only cold rolling applied thereto of 2.1 mm diameter (87.8%), 1.8 mm diameter (total reduction: 91.0%) and 1.3 mm diameter (total reduction 95.3%) were collected as test materials for characterization.

Regarding the above test materials, the following tests were appropriately conducted as described before.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test

2) Measurement test of hardness by Vickers hardness testing machine

3) Measurement test of ferrite grain size (d) by microscopic test

The above test results are shown in Table 10.

TABLE 10 Wire diameter Cold rolling Average ferrite of test total Industrial Tensile Reduction Vickers grain size in C material reduction Strain strength of area hardness C cross section Test (mass %) (mm) (%) e TS (Mpa) RA (%) Hv (−) d (μm) Example 3 0.03 2.1 87.8 2.10 — — — 186 1.8 91.0 2.41 1140 72.3 — — 1.3 95.3 3.06 1202 70.2 310 —

From the above test results, the following is understood.

In Example 3, the tensile strength TS of steel wire is high level of 1140 MPs in wire diameter 1.8 mm (total reduction: 91.0%) and 1202 MPa in wire diameter 1.3 mm (total reduction: 95.3%). The reduction of area RA at this time is high level of 72.3% and 70.2%, respectively. Further, the Vickers hardness HV reaches extremely high level of 310 in wire diameter 1.3 mm (total reduction: 95.3%). Thus, the average ferrite grain size in C directional cross section is fine-grained 186 μm. It is understood that by the cold rolling to a warm rolled material, not only the tensile strength TS is further improved, but the reduction of area RA is maintained high level, and balance between those is excellent. This is due to that the 1.3 mm diameter steel wire of Example 3 has the average ferrite grain size in C directional cross section of 186 nm, and has a ferrite main phase showing the form extended in the direction of the cold wiredrawing working in a bamboo structure shape. The results between Example 3 and Example 1 are compared here. Comparing those on the tensile strength TS and the reduction of area RA in the 1.3 mm diameter (total reduction: 95.3%) which is the same wire diameter in those Examples, Example 3 having low Si content of 0.03 mass % shows the tensile strength TS lower than that of Example 1 having high Si content of 0.30 mass % (Example 3: 1202 MPa and Example 1: 1567 MPa), but those Examples are reversed in the reduction of area RA, and Example 3 is apparently high (Example 3: 70.2% and Example 1: 60.2%). Regarding C content, it is regarded that there is no substantial difference therebetween (Example 3: 0.03 mass % and Example: 0.0245 mass %).

COMPPARATIVE EXAMPLE 1 TO COMPARATIVE EXAMPLE 3

Next, the following tests were carried out as a first group of the Comparative Examples that are outside the scope of the invention of this application.

Test materials for characterization were collected from the commercially available steel wire rods having been completed the working at A3 transformation point or more that is the general hot rolling condition, that are the carbon steel wire rods for cold heading defined in JIS G 3507 and the 6.0 mm diameter steel wire rods having each chemical component composition corresponding to SWRCH5A, SWRCH10A and SWRCH18 to be used in Comparative Examples 1 to 3 as shown in Table 11, and the steel wires after collection of the test materials were continuously applied to the tests in Comparative Examples 1 to 3. Regarding the test material for characterization, the tests of the following items were conducted as described before.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test

2) Measurement test of ferrite grain size (d) by microscopic test

Those test results are shown in Table 12.

TABLE 11 JIS cor- Chemical component composition (mass %) re- spond- ing to Sol. Supplied component C Si Mn P S Al Comparative SWCH5A 0.04 0.04 0.33 0.003 0.007 0.028 Example 1 Comparative SWCH10A 0.09 0.01 0.30 0.011 0.025 0.030 Example 2 Comparative SWCH18A 0.18 0.01 0.79 0.017 0.005 0.040 Example 3

TABLE 12 Average ferrite grain JIS Test material Tensile Reduction of size in C directional corresponding C content Production diameter strength area cross section Supplied to component (mass %) step (mm) TS (MPa) RA (%) d (μm) Comparative SWCH5 0.04 Hot rolling 6.0 350 80.0 20 Example 1 Comparative SWCH10 0.09 430 75.0 17 Example 2 Comparative SWCH18 0.18 520 72.0 16 Example 3

From the above test results, the following is understood. Meanwhile, the test material for characterization is the general hot rolled material, that is, a steel wire rod having been completed rolling working at A3 transformation point or more. This is the production condition of a steel wire rod, outside the scope of the production process of the invention of this application. Consequently, the average grain size in C directional cross section of the ferrite that is the main phase structure of a metal crystal is about from 16 to 20 μm, thus not being a fine-grained structure. For this reason, the reduction of area RA is high level of from 80.1 to 85.9% and is excellent, but the tensile strength TS is from 350 to 550 MPa. Therefore, it is understood that the tensile strength is considerably low as compared with 817 MPa (see Table 9) of the steel wire rod having C content of from 0.0245 to 0.03 mass % produced by warm rolling, supplied to Examples 1 to 3.

Continuously, using the 6.0 mm diameter hot rolled steel wire rod after collection of the above test material for characterization, a steel wire was prepared by cold working up to 1.3 mm diameter with cold wiredrawing or cold rolling, as the steel wire production test in the following Comparative Examples 1 to 3.

Regarding (i) a hot rolled steel wire rod corresponding to SWCH5A in Comparative Example 1, a steel wire was produced by applying cold wiredrawing. The cold wiredrawing was conducted under the same conditions as in Example 1 (See Table 4. Wiredrawing temperature is lower than 200° C.). In this cold wiredrawing step, steel wire test materials of 2.1 mm diameter (wiredrawing total reduction: 87.8%), 1.8 mm diameter (wiredrawing total reduction: 91.0%) and 1.3 mm diameter (wiredrawing total reduction: 95.3%) in the cold wiredrawn form were collected for characterization. Contrary to this, regarding the hot rolled steel wire rods corresponding to SWCH10A of Comparative Example 2 and SWCH18A of Comparative Example 3, steel wires were produced by applying cold rolling. The cold rolling condition is the same as in Example 2 (See Table 6. Rolling temperature is lower than 200° C.). In this cold rolling step, steel wire test materials of 3.3 mm diameter (wiredrawing total reduction: 69.8%), 2.3 mm diameter (wiredrawing total reduction: 85.3%) and 1.3 mm diameter (wiredrawing total reduction: 95.3%) in the cold rolled form were collected for characterization.

The following tests were conducted on the above test materials.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test

2) Recess shaping test of machine screw: This is the same as described before. Regarding Comparative Example 2, the 1.3 mm diameter steel wire in the form of cold rolled form was subjected to the spheroidizing and annealing treatment to prepare a test material having improved cold workability, and the recess shaping test of machine screw was carried out on this material.

3) Torsional torque test of machine screw: This is the same as described above. Regarding materials in which M1.6 pan-head machine screw could be shaped from the 1.3 mm diameter steel wire by cold heading and rolling, the torsional torque test was carried out.

The above test results are shown in Table 13.

TABLE 13 Wire diameter Recess Torsional of test Cold working total Tensile Reduction shapability breaking C material reduction Industrial Spheroidizing strength of area of machine torque Test (mass %) (mm) (%) strain annealing TS (Mpa) RA (%) screw (kgf × cm) Comparative 0.04 2.1 Wiredrawing: 87.8 2.10 None 814 64.0 — — Example 1 1.8 Wiredrawing: 91.0 2.41 None 857 64.7 — — 1.3 Wiredrawing: 95.3 3.06 None 962 64.9 Good 2.35 Comparative 0.09 3.3 Rolling: 69.8 1.20 None 783 — — — Example 2 2.3 Rolling: 85.3 1.92 None 828 64.8 — — 1.3 Rolling: 95.3 3.06 None 1025  62.5 Sometimes 2.43 crack Do — — Good 2.24 Comparative 0.18 3.3 Rolling: 69.8 1.20 None 868 — — — Example 3 2.3 Rolling: 85.3 1.92 None 934 58.0 — — 1.3 Rolling: 95.3 3.06 None 1176  58.9 Crack — Do — — Crack — Cold working total reduction means the total reduction by cold wiredrawing or cold rolling.

The following is understood from the above test results. That is, those test materials are the steel wire test materials obtained in the test course of Comparative Examples 1 to 3 outside the scope of the invention of this application, and C content is the level of from 0.04 to 0.18%. Cold wiredrawing or cold rolling is applied to the steel wire prepared by hot rolling. The tensile strength TS increases and the reduction of area RA decreases, with increasing the total reduction (shown in FIG. 4 and FIG. 5 described hereinafter). It is understood that the total reduction for that the tensile strength TS exceeds 1000 MPa is nearly 95.3% corresponding to the wire diameter 1.3 mm in Comparative Examples 2 and 3. However, the behavior of the reduction of area RA when the tensile strength TS exceeds 1000 MPa incurs great decrease of about 20% of from 85.9-83.0% before cold rolling (see Table 12, Comparative Examples 2 and 3) to 62.5-64.4%. On the other hand, regarding Comparative Example 1, the tensile strength TS remained 962 MPa even in the wire diameter 1.3 mm of the reduction of area of 95.3% because the tensile strength of the cold wiredrawn steel wire rod was low as 350 MPa. Despite of this, the reduction of area RA greatly decreases from 80.1% to 64.9%.

[IV] Comparison and Investigation of Test Result Between Examples 1 to 3 and Comparative Examples 1 to 3 [IV] (1) Regarding Tensile Strength TS and Reduction of Area RA

In any of Examples 1 to 3 and Comparative Examples 1 to 3, the tensile strength TS increases and the reduction of area RA decreases, with increasing the total reduction by cold working. The total reduction R is indicated by the value (according to the above equation (3)) converted into the industrial strain e described above. This is plotted as x axis, and the state of change in the tensile strength TS or the reduction of area RA to the change of the industrial strain e is shown in FIG. 4 and FIG. 5, respectively.

As is apparent from FIG. 4 and FIG. 5, in Examples 1 to 3, the tensile strength TS roughly shows remarkable increase linearly from over 700 to over 800 MPa level in the raw material (6.0 mm diameter steel wire rod: warm rolled material, e=0) to 1200 to 1570 MPa level in the 1.3 mm diameter steel wire (total reduction R=95.3%, e=3.06).

With the rough amount increased of such an extremely large tensile strength TS: 500 to 770 MPa, the reduction of area RA remains about 10% of its rough amount decreased, as from 75 to lower 80% in the raw material to 60 to 75% level in the 1.3 mm diameter steel wire. Contrary to this, in Comparative Examples 1 to 3, the tensile strength TS greatly increases roughly linearly from 350 to 550 MPa level in the raw material (6.0 mm diameter steel wire rod: warm rolled material, e=0) to over 1000 to over 1150 MPa level in the 1.3 mm diameter steel wire (total reduction R=95.3%, e=3.06). With the rough amount increased: 600 to 650 MPa, the reduction of area RA shows its rough amount decreased of about 20%, as from 80 to 85% in the raw material to lower 65 to lower 70% level in the 1.3 mm diameter steel wire. This is large as compared with Examples 1 to 3.

FIG. 6 shows the relationship between the tensile strength TS and the reduction of area RA regarding Examples 1 to 3 and Comparative Examples 1 to 3. The advantage of the strength-ductility balance in the Examples is apparent by this. That is, in the Examples, the tensile strength TS is already in greatly high level in the raw material as compared with the Comparative Examples, and further remarkably increases by cold working. Therefore, high strength exceeding 1500 MPa is obtained. On the other hand, in the Comparative Examples, the tensile strength TS of the raw material remains in the conventional level, and as a result, the tensile strength is at most less than 1200 MPa even by the increase by cold working. Further, in the Examples, extremely great advantages were confirmed that the decrease amount of the reduction of area RA with high strength is remarkably small as compared with the Comparative Examples, and the level of the reduction of area RA after its decrease is still in a level higher than the level in the Comparative Examples. Thus, in the steel wire according to the invention of this application, one that has high strength, maintains ductility in far high level, and is excellent in strength-ductility balance is obtained.

[IV] (2) Shapability of Cross-Shape Recess of M1.6 Pan-Head Machine Screw

On the other hand, according to the recess shapability test, regarding Comparative Examples 2 and 3 having the tensile strength TS exceeding 1000 MPa, Comparative Example 2 in which the test material had already been subjected to spheroidizing annealing treatment does not generate recess crack and is good, but Comparative Example 3 generates crack even though having been subjected to the spheroidizing annealing treatment. In the case that only cold working was conducted and the spheroidizing annealing treatment was not conducted, recess crack occurs in both Comparative Examples 2 and 3. However, in Comparative Example 1 in which the tensile strength is less than 1000 MPa (962 MPa in wire diameter 1.3 mm of total reduction 95.3%), the recess crack is good.

Further, the torsional breaking torque is about 2.3 kgf×cm even in Comparative Example 1 in which recess crack did not occur, and one having been subjected to the spheroidizing annealing treatment in Comparative Example 2, and does not reach the desirable level of 3.0 kgf×cm.

Thus, in the Comparative Examples outside the scope of the invention of this application, when the total reduction in cold wiredrawing or cold rolling to the raw material elevates a certain value or more, crack occurs at the time of recess shaping of M1.6 pan-head machine screw requiring extremely severe cold headability, unless an appropriate softening treatment such as spheroidizing annealing is applied. Contrary to this, in the Examples, it is understood that even when only cold wiredrawing or cold rolling is conducted without conducting spheroidizing annealing, crack does not occur even in the recess shaping requiring such an extremely severe cold headability. Further, from the standpoint of cold workability other than such a particular sold headability, it is understood that even in the case targeting the level of the reduction of area RA, Examples 1 to 3 are superior to Comparative Examples 1 to 3.

Next, comparing Examples 1 to 3 and Comparative Examples 1 to 3 from the point of difference in component of a steel rod, it is understood that according to the process for producing a high-strength steel in accordance with the invention of this application, a steel wire having excellent cold headability that can maintain the tensile strength TS in a high level such as 1000 MPa or more and also the reduction of area RA in a considerably high level such as 65% or more can be obtained in a cold worded state without spheroidizing annealing, using a low carbon steel having C content of about 0.03 mass % as a raw material.

A graph wherein the level of the tensile strength TS to C content of a steel wire is separately indicated between the Examples and the Comparative Examples is shown in FIG. 7, and a graph wherein the level of the reduction of area RA to C content of a steel wire is separately indicated between the Examples and the Comparative Examples is shown in FIG. 8. Here, the case that the wire diameter is 1.3 mm (industrial strain is 3.06) is shown as the example of the case that the cold working rate is a constant condition. According to this, in the Examples, it is understood that even when C content is relatively lower than the Comparative Examples, the tensile strength TS is high, and the reduction of area RA is the equivalent level or more.

COMPARATIVE EXAMPLE

As a second group of the Comparative Examples, a crude screw and a carburizing quenching screw, produced from the commercially available SWCH16A steel wire produced by the conventional technology were used as Comparative Example 4. This screw is M1.6 pan-head machine screw, and its chemical component composition is shown in Table 14.

TABLE 14 JIS Chemical component corresponding composition (mass %) Supplied to component C Si Mn P S sol. Al Compar- SWCH16A 0.16 0.04 0.74 0.005 0.008 0.030 ative Example 4

The production process of this M1.6 pan-head machine screw is according to the conventional technology, and there are the following two kinds. One is that a steel wire rod is produced by hot rolling, the rod is subjected to cold wiredrawing by the conventional technology to produce a 1.3 mm diameter steel wire, this is subjected to spheroidizing annealing treatment to improve cold headability, and the material is shaped into M1.6 pan-head machine screw by cold heading and rolling (a crude screw), and another is that the crude screw is subjected to carburizing quenching and tempering treatment to shape M1.6 pan-head machine screw having a predetermined strength imparted thereto (a carburizing quenching screw).

As the characterization test of Comparative Example 4, the torsional torque test (the test method is as described before) was conducted using the crude screw and the carburizing quenching screw as test materials. The test results are shown in Table 15.

TABLE 15 Torsional JIS C Vickers breaking corresponding content Kind of Thermal hardness torque Test to component (mass %) screw refining Hv (—) (kgf × cm) Comparative SWCH16A 0.16 M1.6 Crude — 1.82 Example 4 pan-head Carburizing 330 2.96 machine quenching and screw tempering

The following is understood from the above test results. That is, in Comparative Example 4 produced by the production process outside the scope of the invention of this application, regarding the crude screw test material, the torsional torque of M1.6 pan-head machine screw was a low value of 1.82 kgf·cm, but regarding the carburizing quenching screw, high torsional strength of 2.96 kgf·cm is obtained, thus having the desired torsional strength.

In the torsional torque test conducted in Comparative Examples 1 and 2 described before, the torsional breaking torque was low level of from 2.25 to 2.43 kgf·cm, but in Examples 1 and 2 described before, it was 3.38 kgf·cm and from 2.923.38 kgf·cm, respectively. The level of the torsional breaking torque of those Examples is the same level as the level of Comparative Example 4 that is the commercially available product, and each nearly satisfies the desired torsional breaking torque of 3.0 kgf·cm.

From the above test, the industrial advantage of the excellent cold-workability exhibiting high-strength steel wire or steel bar of the invention of this application, and the industrial advantage of the production process for producing those were confirmed.

[V] Examples<Carbon steel to low alloy steel, wherein main phase of metal crystal structure is substantially cementite-free, C content is solid solution limit or less of carbon in ferrite phase at Ael point, or C content is 0.010 mass % or less> [V]-(1) Common Test Procedures Between Examples 1 to 5 and Examples 6 to 9. Examples 1 to 9 being fallen within the scope of the invention of this application were tested as follows.

Each of steels having the chemical component composition of component Nos. 1 to 5 shown in Table 16 was melted using a vacuum melting furnace, and cast into a steel ingot. The compositional characteristics here are that it is an ultra-low carbon steel in which carbon C is changed within a range of C content of from 0.0014 to 0.0109 mass %, component No. 4 has high level of Si=1.01 mass % as compared with others, and component No. 5 has N=0.0080 mass % which is relatively high as compared with others.

TABLE 16 Chemical component Component composition (mass %) No. Supplied C Si Mn P S N Sol. Al 1 Examples 0.0014 0.30 0.20 0.009 0.001 0.0031 0.028 1 and 6 2 Examples 0.0047 0.30 0.20 0.009 0.001 0.0024 0.026 2 and 7 3 Examples 0.0098 0.30 0.20 0.009 0.001 0.0025 0.028 3 and 8 4 Examples 0.0109 1.01 0.19 0.010 0.001 0.0020 0.033 4 and 9 5 Example 5 0.0095 0.30 0.19 0.009 0.001 0.0080 0.029

The steel ingots obtained were shaped into a steel bar of 80 mm square. Metal structure of those steel bars comprises a ferrite, and the average grain size of the ferrite in C directional cross section was about 20 μm or less. A material for rolling was collected from the above each steel bar of 80 mm square, shaped into 18 mm square by warm multidirection and multipass caliber rolling and cooled with water to prepare a steel bar. This warm rolling is to prepare the raw material for the steel wire or steel bar according to the invention of this application, and was conducted under the condition that the average crystal grain size in a cross section perpendicular to the longitudinal direction of the material obtained by the warm rolling is 3 μm or less.

As the warm caliber rolling method in which the average crystal grain size is 3 μm or less as described above, the method was conducted under the following conditions. The 80 mm square raw material for rolling shaped by the above hot forging was heated to 550° C. Thereafter, in the rolling temperature range of from 450 to 530° C., the warm rolling of 19 passes in which the reduction of each one pass is about 17% was conducted by the diamond style caliber roll (see FIG. 2, the upper figure) as shown in Table 2 to shape into 24 mm square. Next, the warm rolling was conducted by the oval style caliber roll having the maximum short axis length of 11 mm and a long axis length of 52 mm (a and b in FIG. 2, the lower figure, provided R=64 mm), and finally the one pass warm rolling was conducted by the square style caliber roll, thereby shaping into 18 mm square with the total 21 passes. The total reduction from the warm rolling raw material (80 mm square) to this 18 mm square material is 95%. The outline of pass schedule was shown in Table 2.

In the one pass warm rolling by the above oval style caliber roll, the 24 mm square bar is subjected to rolling by the above oval style caliber roll. Therefore, the proportion of the maximum short axis length 11 mm in C directional cross section of the material after rolling to the opposite side length 24 mm in C directional cross section of the material before rolling is considerably small as (11 mm/24 mm)×100=46%, and the reduction calculated from the hole dimension at this time is considerably large as 38%. Therefore, the one pass warm rolling by the oval style caliber roll constitutes the condition that further promotes fine-graining of the ferrite grain size in the 18 mm square steel bar after completion of warm rolling. In the rolling course by the diamond style caliber roll up to the 19th pass, the rolling that continuously passes through the same caliber roll every two passes (a so-called “double pass”) is appropriately carried out in order to approach the cross sectional shape of the material to a square as possible, and each double pass was counted as two passes, respectively.

Further, the material was rotated around the lengthwise direction axis core every each pass of rolling to change the rolling direction, and the multipass rolling in multidirection was conducted. Further, by virtue of shaping exothermic heat, heat release amount is relatively small in the relatively low temperature side region even in the rolling temperature region of warm rolling, and there was no necessity of intermediate heating due to temperature lowering of the rolling material. Next, the 18 mm square steel bar prepared by the above warm rolling method was reduced in its diameter by machiening, thereby shaping into a 6.0 mm diameter wire rod.

The reason that the diameter is reduced from 18 mm square to 6.0 mm diameter is as described below. In this Example, M1.6 pan-head machine screw (a diameter in the effective cross section in a screw portion is 1.27 mm) defined in JIS B1111 as the use of a steel wire was selected. Therefore, it is intended to prepare a material by which 1.3 mm diameter is obtained by the cold wiredrawing working with the wiredrawing rate 95% as the target or the cold rolling working with the total reduction 95% as the target. The reason for selecting M1.6 pan-head machine screw is as follows. To form a cross-shape recess (depressed portion giving torque with a screw driver) on the top of the screw by heading, an extremely excellent cold headability is required. Therefore, it is for the evaluation as to whether having remarkably excellent cold headability, by a cross-shape “recess shaping test” of M1.6 pan-head machine screw described hereinafter.

In the above, the grain size in C directional cross section of the 18 mm square steel bar prepared by warm rolling was equalized over the entire surface.

Those 6.0 mm diameter test materials for characterization (hereinafter referred to as “AO group test material”, and the constitution number comprises 5 kinds corresponding to component Nos. 1 to 5 in Table 16) were collected, and the tests of the following items were carried out.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test: This test has the object to obtain basic data as to whether it is a material having high level balance between strength and cold workability, such that strength is particularly excellent, and simultaneously cold workability is also considerably excellent.

2) Harness measurement test by Vickers hardness testing machine: This test is effective to confirm correlativity with tensile strength, as one of strength characteristics, and in the case that collection of a tensile test specimen is difficult. This test was conducted based on the method defined in JIS Z 2244.

3) Measurement test of ferrite grain size (d) by microscopic test: An appropriate microsection is prepared from each test material, and as an average grain size of a ferrite constituting the main phase by a microstructure of a metal crystal, an average ferrite grain size in a cross section (C directional cross section) in a direction perpendicular to the longitudinal direction of the test material (consistent with the longitudinal direction of the above 18 mm square steel bar) is measured. In such a case, actually the average ferrite grain size in C directional cross section was obtained by observing the microstructure in L directional cross section. Hereinafter the same is applied in this description.

The above test results relating to the warm rolling material are shown in Table 17.

TABLE 17 Wire diameter Grain of test Tensile Reduction Vickers size of Component C Production material strength of area hardness ferrite Test material No. Supplied (mass %) step (mm) TS (Mpa) RA (%) Hv (—) d (μm) A0 group 1 Examples 1 0.0014 Warm 6.0 635 81.9 221 0.9 (Warm rolled and 6 rolling Material) 2 Examples 2 0.0047 665 80.0 226 0.8 and 7 3 Examples 3 0.0098 795 78.1 234 0.7 and 8 4 Examples 4 0.0109 760 80.7 233 0.7 and 9 5 Example 5 0.0095 710 80.1 210 0.7

The following is understood from the test results in Table 17. The AO group test materials are confirmation test materials of raw materials supplied to cold working conducted in Examples 1 to 9. The AO group test materials are materials prepared by warm rolling satisfying the preparation condition (production condition) of a raw material (a steel wire rod) in the structural elements of the process for producing the excellent cold-workability exhibiting high-strength steel wire or steel bar according to the invention of this application, and the chemical component composition of the raw material has metallographically cementite-free carbon steel component. Therefore, fine particles wherein the microstructure of metal crystal is cementite-free and the average ferrite grain size is from 0.7 to 0.9 μm are obtained. Due to this, high strength that the tensile strength TS is 635 MPa or more is secured, and simultaneously, the characteristic of high level that the reduction of area RA is 78% or more is obtained. Thus, it is understood to be a raw material having excellent balance between strength and shapability. The process for producing the excellent cold-workability exhibiting high-strength steel wire or steel bar according to the invention of this application, and the product obtained thereby are obtained by subjecting cold working to the raw material having such material characteristics. In particular, it is understood to have high level that the tensile strength is 600 MPa or more, even in ultra-low carbon steel such that C content is 0.0014 to 0.0109 mass % or less.

[V] (2) Each Test in Examples 1 to 5 and Examples 6 to 9

Using each steel wire rod of 6.0 mm diameter after collecting the AO group test material, a test for producing a steel wire by cold working from 6.0 mm diameter up to 1.3 mm diameter was carried out by cold wiredrawing in Examples 1 to 5 and by cold rolling in Examples 6 to 9.

[V] (2)-i) Example 1 to Example 5 (Production Test of Steel Wire by Cold Wiredrawing)

Tests for producing steel wires were conducted by using five kinds of 6.0 mm diameter steel wire rods of component Nos. 1 to 5 (see Table 16) prepared by warm rolling as described before as raw materials, and wiredrawing up to 1.3 mm diameter by cold wiredrawing (hereinafter referred to as “Example 1 to Example 5”, respectively). Conditions of cold wiredrawing in those Examples are all as follows. That is, the 6.0 mm diameter steel wire rod at ordinary temperature (a steel wire rod prepared by shaping into 18 mm diameter by warm rolling, and then cut-working into 6.0 mm diameter, as described before) was successively wiredrawn by wiredrawing dies of die No. 1 to No. 17 to produce 1.3 mm diameter steel wire. The material temperature during wiredrawing was less than 200° C.

TABLE 18 Cold wiredrawing in Examples 1 to 5 Die No. 1 to 11 12 to 13 14 to 17 Steel wire diameter after 5.6*) to 2.1 1.9 to 1.8 1.6 to 1.3 wiredrawing (mm) Wire diameter of test material 2.1 1.8 1.3 (mm) Wiredrawing total reduction in 87.8 91.0 95.3 collection stage of test material (%) Industrial strain in collection stage 2.10 2.41 3.06 of test material e (%) Note *)Starting diameter of a raw material for wiredrawing is 6.0 mm.

In the wiredrawing step of all of those Examples, wiredrawing of from 6.0 mm diameter up to 1.3 mm could easily be conducted without applying spheroidizing annealing and other softening treatment at all. During this, in each stage of 2.1 mm diameter (wiredrawing total reduction: 87.8%), 1.8 mm diameter (wiredrawing total reduction: 91.0%) and 1.3 mm diameter (wiredrawing total reduction: 95.3%), the wiredrawn test material for characterization (hereinafter referred to as “Al group test materials”) was collected. The Al group test materials are five kinds of Examples 1 to 5, each kind having three levels in wire diameter, and therefore comprise the total of 5 kinds×3=15 kinds. Further, regarding the 1.3 mm diameter test material, the test of cold working into M1.6 pan-head machine screw was conducted. Regarding the test materials “A1 group test materials” of Example 1 to Example 5, the tests of the following items were carried out.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test (same as described before)

2) Hardness measurement test by Vickers hardness testing machine (same as described before)

3) Measurement test of average ferrite grain size (d) by microscopic test (same as described above)

4) Recess formation test of machine screw: This was conducted on only a steel wire of wire diameter 1.3 mm. As described before, a steel wire of wire diameter 1.3 mm was preliminarily shaped by header working in the production step of M1.6 pan-head machine screw defined in JIS B1111, and the predetermined cross-shape recess (depressed portion such as cross-shape for screwing the screw by a screw driver) is formed on the top by cold heading. It is a test of observing the state that cracks generate on this recess at the time of shaping, with a magnifying glass of 10 magnifications. In general, generation state of recess crack greatly varies depending on the recess shape of a machine screw, but a cross-shaped recess formation on M1.6 pan-head machine screw is extremely severe head-shaping. In the present specification, it was positioned that this test is a practical test, and at the same time, particularly excellent evaluation test for cold headability. The state that crack was not observed was indicated “Good”, the state that fine cracks were observed, but it was good as a whole was indicated “Slightly good”, the state that cracks were observed was indicated “Crack”, and the state that great cracks occurred was indicated “Great crack”.

5) Torsional torque test of machine screw: A screw portion is formed by cold rolling on a screw intermediate having the above-described recess head-shaped from a steel wire of wire diameter 1.3 mm to prepare M1.6 pan-head machine screw. Torque is increased in this until breakage of the screw by an appropriate torque measurement device according to the method defined in 5.4 “Torsional test” of JIS B 1060 “Mechanical properties and performance of meter type thread rolling screw having carburizing, quenching and tempering applied thereto”. The torque value required to induce breakage (breaking torque (kgf·cm)) was measured. The object of this test is to evaluate “Torsional strength” that is one of characteristics of mechanical properties to a fixing part such as a screw or a bolt. Hereinafter the same in the present specification. In the case of M1.6 pan-head machine screw, it is desirable that the breaking torque is 3.0 kgf·cm or more.

6) Torsional delay fracture test of machine screw: M1.6 pan-head machine screw prepared from a steel wire of wire diameter 1.3 mm was closed and set in a state of twisting the test piece as shown in FIG. 9 at a value of 70% of a breaking torque value obtained by the breaking torque test, and delay fracture resistance characteristic was evaluated as to whether torsional breaking generates in 72 hours. Set number of torsional test pieces is 10. This torsional delay fracture test was carried out on only Example 2. The above test results are shown in Table 19 and Table 20.

TABLE 19 Wire diameter of Wiredrawing Tensile Reduction Component C test material total reduction strength of area Test material Test No. (mass %) (mm) (%) Strain TS (Mpa) RA (%) A1 group Example 1 1 0.0014 2.1 87.8 2.10 1070 81.2 Example 2 2 0.0047 1125 78.7 Example 3 3 0.0098 1214 73.1 Example 4 4 0.0109 1252 73.1 Example 5 5 0.0095 1220 74.5 A1 group Example 1 1 0.0014 1.8 91.0 2.41 1142 76.6 Example 2 2 0.0047 1199 76.1 Example 3 3 0.0098 1247 73.9 Example 4 4 0.0109 1322 69.8 Example 5 5 0.0095 1272 72.1

TABLE 20 Wire Wiredrawing diameter of total Tensile Reduction Vickers Test Component C test material reduction strength of area hardness material Test No. (mass %) (mm) (%) Strain TS (Mpa) RA (%) Hv (—) A1 group Example 1 1 0.0014 1.3 95.3 3.06 1370 71.8 313 Example 2 2 0.0047 1419 70.2 334 Example 3 3 0.0098 1539 70.2 357 Example 4 4 0.0109 1568 62.1 369 Example 5 5 0.0095 1521 67.6 355 Wire diameter Average grain size in C Torsional of test directional cross Recess shapability of breaking material section of ferrite M1.6 pan-head torque Torsional delay Test (mm) d (nm) machine screw (kgf × cm) fracture Example 1 1.3 138 Good 2.93 — Example 2 150 Good 3.07 No fracture Example 3 175 Slightly good 3.27 — Example 4 140 Great crack 3.26 — Example 5 161 Crack 3.28 —

The following is understood from the test results in Table 19 and Table 20. That is, the A1 group test materials are all test materials collected from the steel wires obtained by the Examples belonging to the scope of the invention of this application. In more detail, the Al group test materials are that the component has ultra-low C content (C: 0.0014 to 0.0109%) and is the cementite-free fine ferrite crystal (average grain size d≦0.9 μm) as described above, the level of the tensile strength TS and reduction of area RA is high, and cold wiredrawing by the cold wiredrawing reduction of 88% or more is subjected to the such a raw material (a steel wire rod) having its excellent balance. Therefore, in any of Example 1 to Example 5, the steel wire obtained remarkably increases its tensile strength TS with increasing the total reduction by cold wiredrawing. Despite of this, the decrease amount of the reduction of area RA is abnormally small. This state is shown in FIG. 10 and FIG. 11 (the results of Comparative Example 1 to Comparative Example 3 described hereinafter are also shown in both drawings). Totally considering those drawings, it becomes apparent. In FIG. 10 and FIG. 11, the total reduction R due to cold wire drawing was indicated in the horizontal axis in terms of the value converted into the industrial strain e described before (according to the above equation (3)). Further, the industrial strain e was indicated in those Tables. Hereinafter the same as above.

As is apparent from this, the tensile strength remarkably increases from 635-795 MPa level of the raw material to 1070-1252 MPa level at the wiredrawing total reduction of 87.8%, to 1142-1322 MPa level at the wiredrawing total reduction of 91.0%, and to 1370-1568 MPa level at the wiredrawing total reduction of 95.3%. Despite of such a remarkable increase of the tensile strength TS, the decrease amount of the reduction of area RA is abnormally small. That is, the level of 78.1-81.9% in the raw material before wiredrawing decreases 73.1-81.2% level at the wiredrawing total reduction of 87.8%, 69.8-76.6% level at the wiredrawing total reduction of 91.0%, and 62.1-71.8% level at the wiredrawing total reduction of 95.3%, but the decrease amount is abnormally small. In addition, because of being cementite-free, any softening treatment such as spheroidizing annealing is not applied in the step during this.

Further, examining the relationship between the industrial strain E and the tensile strength from those drawings, it is understood that in the raw material, the tensile strength TS is already in a high level of 635-795 MPa level, and the tensile strength further increases even by slight strain. For example, according to Example 3, it is understood that a product having high strength exceeding 800 MPa at C content of 0.0095 mass % is obtained by cold working of industrial strain ε=0.17. Because the wiredrawing total reduction R when ε=0.17 is calculated as 17%, the wire diameter of the steel wire at this time is 5.5 mm. In the present Example, because the diameter of a raw material just before cold wiredrawing (corresponding to the diameter of a steel wire rod) is set to be 6.0 diameter, it is possible to produce a steel wire exceeding 800 MPa even in rather large wire diameter of 5.5 mm or more by setting the diameter further large, and the reduction of area at that time is secured to exceed 75%. From the result of the above test, the relationship between the tensile strength TS and the reduction of area RA is shown in FIG. 12. According to the drawing, it is understood that it is possible to produce a high-strength steel or steel bar having excellent balance between strength and ductility so as to (1) secure TS≧1000 MPa and RA≧70%, (2) secure TS≧1200 MPa and RA≧65%, or (3) secure TS≧1500 MPa and RA≧60%.

Thus, it is understood that the above-described excellent cold-workability exhibiting high-strength steel according to the invention of this application is in a cold wiredrawn state, and the above material characteristics are obtained in a steel wire to which thermal refining such as quenching and tempering has not been applied. The crystal structure of the steel wire having such excellent material characteristics is a cementite-free ferrite showing a form extended in a bamboo structure state in the cold wiredrawing working direction, and the average ferrite grain size in C directional cross section of the steel wire having a wire diameter of 1.3 mm is ultrafine particles of from 138 to 175 nm (see FIG. 20). Structure photograph by TEM (transmission electron microscope) regarding Example 2 is illustrated in FIG. 13. The average ferrite grain size is 150 nm. From the conception that the ferrite grain size in C directional cross section after the cold working is controlled by the working strain amount, the investigation is made from the measurement value of the grain size before and after the cold working. For example, in the case of Example 2, the average ferrite grain size in C directional cross section in the steel wire rod prepared by warm rolling (a steel wire rod just before initiation of cold working) was 0.8 μm (see Table 17).

The average ferrite grain size (=d2) in C directional cross section of the steel wire having the chemical component composition in the present Example and the production history of this steel wire rod is calculated by the following equation (5):

d2=(1−R/100)½×d1  (5)

wherein R: Total cross section reduction by cold working (%)

d1: Estimated from ferrite grain size in C directional cross section just before initiation of cold working

R is calculated by the total reduction of from the steel wire rod having a wire diameter of 6.0 mm into the steel wire having a wire diameter of 1.3 mm, and R=95.3% is obtained. Because d1 is 0.8 μm, it is calculated as d2=173 nm.

This calculated value 173 nm well consists with the measured value 150 nm.

Therefore, in the process for producing the steel wire or steel bar according to the invention of this application, in producing the steel wire by cold working of the steel wire rod as a warm rolled material, it is effective to use the above equation (5) as the controlling means of the ferrite grain size in C directional cross section of the steel wire rod. The steel wire thus produced according to the invention of this application is quite good in Example 1 and Example 2, and substantially reaches the level having no problem in Example 3, against the recess formation that is the formation stage to which extremely severe cold heading, such as M1.6 pan-head machine screw, is applied in the state without the thermal refining such as quenching and tempering. It is understood that M1.6 pan-head machine screw shaped from the steel wire having such an excellent cold headability by the cold working method of cold heading and cold rolling has high torsion strength of about 3.0 kgf·cm as its torsional breaking torque.

[V] (2)-2) Example 6 to Example 9 (Production Test of Steel Wire by Cold Rolling)

Similarly, tests for producing steel wires were conducted by using four kinds of 6.0 mm diameter steel wire rods of component Nos. 1 to 4 (see Table 16) prepared by warm rolling as described before as raw materials, and wiredrawing up to 1.3 mm diameter by cold wiredrawing (hereinafter referred to as “Example 6 to Example 9”, respectively). The production process of the steel wire differs in that the warm rolled steel wire rod was subjected to cold wiredrawing in Example 1 to Example 5, but the same warm rolled steel wire rod was subjected to cold rolling in those Example 6 to Example 9. The conditions of the cold rolling are all as follows. The 6.0 mm diameter steel wire rod at ordinary temperature (a steel wire rod prepared by working into 18 mm diameter by warm rolling, and then machiening into 6.0 mm diameter, as described before) was cold rolled by each combined roll in the first step to the third step.

TABLE 21 Cold rolling in Examples 6 to 9 Rolling step No. First step Second step Third step Number of pass 8 10 5 Steel wire diameter after cold 5.7*) to 3.3 3.1 to 1.8 1.6 to 1.3 rolling (mm) Wire diameter of test material 3.3 2.3, 1.8 1.3 (mm) Rolling total reduction in 69.8 85.3, 91.3 95.3 collection stage of test material (%) Industrial strain in collection 1.20 1.92, 2.41 3.06 stage of test material e (%) Note *)Starting diameter of a raw material for rolling is 6.0 mm.

That is, a steel wire was produced by rolling from 6.0 mm diameter to 3.3 mm diameter by 8 passes in the first step, rolling from 3.3 mm diameter to 1.8 mm diameter by 10 passes in the second step, and rolling from 1.8 mm diameter to 1.3 mm diameter by 5 passes in the third step. The material temperature during rolling was lower than 200° C. In the rolling steps of all of those Examples, cold rolling could be carried out from 6.0 mm diameter up to 1.3 mm diameter without applying any other softening treatment such as spheroidizing annealing. During this, as the test materials for characterization, the steel wire test materials in a rolled state (hereinafter referred to as “A2 group test materials”) were collected in the four stages of 3.3 mm diameter (total reduction: 69.8%), 2.3 mm diameter (total reduction: 85.3%), 1.8 mm diameter (total reduction: 91.0%), and 1.3 mm diameter (total reduction: 95.3%). The A2 group test materials are four kinds of Examples 6 to 9, each kind having four levels in wire diameter, and therefore comprise the total of 4 kinds×4=16 kinds. Further, regarding the 1.3 mm diameter test material, the test of cold working into M1.6 pan-head machine screw was conducted.

Regarding the test materials “A2 group test materials” of Example 6 to Example 9, the tests of the following items were carried out.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test (same as described before)

2) Hardness measurement test by Vickers hardness testing machine (same as described before)

3) Recess formation test of machine screw: Only the 1.3 mm diameter steel wire (same as described above)

4) Torsional torque test of machine screw: Only M1.6 pan-head machine screw (same as described above)

The above test results are shown in Table 22 and Table 23.

TABLE 22 Rolling Wire diameter total Tensile Reduction of Component C of test material reduction strength area Test material Test No. (mass %) (mm) (%) Strain TS (Mpa) RA (%) A2 group Example 6 1 0.0014 3.3 69.8 1.20 773 — Example 7 2 0.0047 836 — Example 8 3 0.0098 895 — Example 9 4 0.0109 999 — A2 group Example 6 1 0.0014 2.3 85.3 1.92 875 83.2 Example 7 2 0.0047 968 82.7 Example 8 3 0.0098 1001 76.9 Example 9 4 0.0109 1094 78.5

TABLE 23 Wire diameter Rolling of test total Tensile Reduction Test Component C material reduction strength of area material Test No. (mass %) (mm) (%) Strain TS (Mpa) RA (%) A2 group Example 6 1 0.0014 1.8 91.0 2.41 961 — Example 7 2 0.0047 1050 — Example 8 3 0.0098 1071 — Example 9 4 0.0109 1232 — Wire diameter Rolling Recess Torsional Com- of test total Tensile Reduction Vickers shapability of breaking Test ponent C material reduction strength of area hardness M1.5 pan-head torque material Test No. (mass %) (mm) (%) Strain TS (Mpa) RA (%) Hv (—) machine screw (kgf × cm) A2 Example 6 1 0.0014 1.3 95.3 3.06 1142 78.7 263 Good 2.63 group Example 7 2 0.0047 1276 80.1 294 Good 2.92 Example 8 3 0.0098 1331 72.2 299 Slightly good 2.92 Example 9 4 0.0109 1462 64.0 352 — 3.21

The following is understood from the test results in Table 22 and Table 23. That is, the A2 group test materials are all test materials collected from the steel wires obtained by the Examples belonging to the scope of the invention of this application. Further, the steel wire rods that are raw materials of steel wires are all the same as Examples 1 to 5, and are the materials that are produced by an appropriate warm rolling, have extremely low C content (C: 0.0014 to 0.0109 mass %), have crystals being cementite-free fine ferrite particles (average grain size d=0.7 to 0.9 μm), and have high level of the tensile strength TS and the reduction of area RA with good balance therebetween. Cold rolling is applied to such materials with the rolling total reduction of 69.8% (in the case of 6.0 mm diameter→3.3 mm diameter) or more.

Thus, the point that the production condition of Example 6 to Example 9 differs from that of Example 1 to Example 5 is that the working was conducted by cold rolling in place of cold wiredrawing. Material characteristics of the steel wire thus obtained are shown in FIG. 10, FIG. 11 and FIG. 12 previously described. As is understood from those, the tensile strength TS of the steel wire obtained remarkably increases with increasing of the total reduction by cold rolling.

In addition, despite that the tensile strength TS remarkably increases, the decrease amount of the reduction of area RA is abnormally small. This change of material characteristics is the same in any of Example 6 to Example 9, and is also similar to the results of Example 1 to Example 5. Further, it is understood that the tensile strength TS and the reduction of area RA of the cold rolled steel wire maintain high level, and those are provided with good balance.

It is understood that the advantage of such material characteristics is obtained in the cold rolled state as it is, without applying thermal refining such as quenching and tempering. Further, it was confirmed that even in the cold rolled state as it is without applying spheroidizing annealing, the recess formation of M1.6 pan-head machine screw can be conducted in Example 6, Example 7 and Example 8 each having low C content, and it is extremely excellent in cold workability. The material characteristics conform to Examples 1 to 3.

Further, in Example 7 and Example 8 having such a material characteristic level, the torsional breaking torque exhibits an excellent high torsional strength of about 3.0 kgf·cm even in the state without thermal refining such as quenching and tempering after shaping into M1.6 pan-head machine screw. Moreover, it is thus understood that from the comparison between the result of Examples 1 to 4 and the result of Examples 6 to 9, in the process for producing an excellent cold-workability exhibiting high-strength steel according to the invention of this application, either of cold wiredrawing and cold rolling may be used as the cold working method to the warm rolled steel wire material.

[V] (3) Comparative Example

Next, the following tests were conducted as the Comparative Examples that are outside the scope of the invention of this application. The Comparative Examples were separated into a first group and a second group.

[V] (3) (a) First Group of Comparative Examples (Comparative Example 1 to Comparative Example 3)

As a first group of the Comparative Examples, test materials for characterization (hereinafter referred to as “BO group test materials”) were collected from the commercially available steel wire rods having been completed the working at A3 transformation point or more that is the general hot rolling condition, that are the carbon steel wire rods for cold rolling defined in JIS G 3507 and the 6.0 mm diameter steel wire rods having each chemical component composition corresponding to SWRCH5A, SWRCH10A and SWRCH18 as shown in component Nos. 6 to 8 of Table 24, and the tests of the following items were conducted.

1) Measurement test of tensile strength (TS) and reduction of area (RA) by tensile test (same as described before)

2) Measurement test of ferrite grain size (d) by microscopic test (same as described before)

Those test results are shown in Table 24 and Table 25.

TABLE 24 JIS Chemical component Component corresponding composition (mass %) No. Supplied to component C Si Mn P S Sol. Al 6 Comparative SWCH5A 0.04 0.04 0.33 0.003 0.007 0.028 Example 1 7 Comparative SWCH10A 0.09 0.01 0.30 0.011 0.0025 0.030 Example 2 8 Comparative SWCH18A 0.18 0.01 0.79 0.017 0.005 0.040 Example 3

TABLE 25 Average ferrite Test grain size in C JIS material Tensile Reduction directional cross Test Component corresponding C content Production diameter strength of area section material No. Supplied to component (mass %) step (mm) TS (MPa) RA (%) d (μm) C0 group 6 Comparative SWCH5 0.04 Hot rolling 6.0 350 80.0 20 (hot Example 1 rolled 7 Comparative SWCH10 0.09 430 75.0 17 Material) Example 2 8 Comparative SWCH18 0.18 520 72.0 16 Example 3

The following is understood from the test results of Table 24 and Table 25. That is, the BO group test materials are test material for characterization of raw materials supplied to cold working conducted in Comparative Examples 1 to 3. The BO group test materials are materials (steel wire rods) produced by hot rolling that is the preparation condition of a raw material in the production process of a steel that is outside the scope of the invention of this application. Therefore, the average grain size in C directional cross section of the ferrite that is the main phase structure of metal crystal is from 16 to 20 μm. It is understood that this is extremely large as compared with the average ferrite grain size (from 0.7 to 0.9 μm) of the material used as the steel wire rod in Examples 1 to 9.

For this reason, despite that C content is remarkably high as compared with Example 1 to Example 9, the reduction of area RA is high level of from 80.1 to 85.9%, and is excellent. However, despite that C content is high as such, the tensile strength is from 350 to 550 MPa, and it is understood that this is remarkably low as compared with the tensile strength TS of from 635 to 795 MPa of the steel wire rods used in Examples 1 to 9. On the other hand, using the above hot rolled steel wire rod of 6.0 mm diameter after collecting the above BO group test materials, a steel wire cold worked up to 1.3 mm diameter by cold wiredrawing or cold rolling was prepared.

(1) Regarding the hot rolled steel wire rod of component No. 6 (corresponding to SWCH5A), a steel wire was produced by applying cold wiredrawing. The cold wiredrawing was conducted under the same conditions as in Examples 1 to 5 (see Table 18. The wiredrawing temperature is lower than 200° C.). This is designated “Comparative Example 1”. In this cold wiredrawing step, steel wire test materials in the cold wiredrawn state as it is, having 2.1 mm diameter (wiredrawing total reduction: 87.8%), 1.8 mm diameter (wiredrawing total reduction: 91.0%), and 1.3 mm diameter (wiredrawing total reduction: 95.3%) were collected as for characterization.

(2) Contrary to this, regarding the hot rolled steel wire rods of component No. 7 (corresponding to SWCH10A) and component No. 8 (corresponding to SWCH18A), the steel wires were produced by applying cold rolling. The cold rolling conditions are the same as in Examples 6 to 9 (see Table 21. Rolling temperature is lower than 200° C.). In this cold rolling step, steel wire test materials in the cold rolled state as it is, having 3.3 mm diameter (wiredrawing total reduction: 69.8%), 2.3 mm diameter (wiredrawing total reduction: 85.3%), and 1.3 mm diameter (wiredrawing total reduction: 95.3%) were collected as for characterization. Those tests are designated “Comparative Example 2” and “Comparative Example 3”, respectively. The test materials of Comparative Examples 1 to 3 are put together and designated “B1 group test materials”. The following tests were conducted on those test materials.

1) Measurement test of tensile strength (TS) and reduction of area (RA) (same as described before)

2) Recess formation test of machine screw (same as described above): regarding the steel wire of 1.3 mm diameter, recess formation test of machine screw was conducted on the test material in a cold rolled state. Further, regarding the steel wires of 1.3 mm diameter in Comparative Examples 2 and 3, the test material having improved cold workability was prepared by applying spheroidizing annealing, other than the steel wire test material in a cold rolled state, and the recess formation test of M1.6 pan-head machine screw was conducted on this.

3) Torsional torque test of machine screw (same as described before): With respect to the material that M1.6 pan-head machine screw could be shaped from 1.3 mm diameter steel wire by cold heading and rolling, the torsional torque test was conducted.

The above test results are shown in Table 26.

TABLE 26 Recess shapability Diameter Cold working of M1.6 Tortional Com- of test total Tensile Reduction pan-head breaking Test ponent C material reduction Spheroidizing strength of area machine torque material Test No. (mass %) (mm) (%) Strain annealing TA (Mpa) RA (%) screw (kgf × cm) C1 group Comparative 6 0.04 2.1 Wiredrawing 2.10 None 814 64.0 — — Example 1 87.8 1.8 Wiredrawing 2.41 None 857 64.7 — — 91.0 1.3 Wiredrawing 3.06 None 962 64.9 Good 2.35 95.3 Comparative 7 0.09 3.3 Rolling 1.20 None 783 — — — Example 2 69.8 2.3 Rolling 1.92 None 828 64.8 — — 85.3 1.3 Rolling 3.06 None 1025  62.5 Sometimes 2.43 95.3 crack Do — — Good 2.24 Comparative 8 0.18 3.3 Rolling 1.20 None 868 — — — Example 3 69.8 2.3 Rolling 1.92 None 934 58.0 — — 85.3 1.3 Rolling 3.06 None 1176  58.9 Crack — 95.3 Do — — Crack —

The following is understood from the test results of Table 26 (Comparative Examples 1 to 3). B1 group test materials are the steel wire test materials obtained in the test stage of Comparative Examples 1 to 3 that are outside the scope of the invention of this application, and C content is the level of from 0.04 to 0.18 mass %. When cold wiredrawing or cold rolling is applied to the raw material (steel wire rod) prepared by hot rolling, the tensile strength TS increases and the reduction of area RA decreases, with increasing of its total reduction. The total reduction for exceeding the tensile strength TS of 1000 MPa is achieved in 95.3% corresponding to the wire diameter 1.3 mm in Comparative Examples 2 and 3. The reduction of area RA at this time is decreased to 64.4-66.2%. The decrease state from the material of this reduction of area RA is about 20% decrease of 85.9-83.0%-64.4-62.5%, and its decrease amount is remarkably large. Further, the level of the reduction of area RA value after decrease is considerably low level as compared with RA: about 70-75% (see FIG. 12) when the tensile strength TS exceeds 1000 MPa in Examples 1 to 9.

Thus, the change tendency of the material characteristics that the tensile strength increases with increasing of the total reduction in cold working to a raw material, whereas the reduction of area RA decreases is the same as in the case of Examples 1 to 9 even in Comparative Examples 1 to 3. However, considering quantitatively, the decrease amount of the reduction of area RA in such a case was remarkably small in the case of Examples 1 to 9 ((6 mm diameter warm rolled material: 78.1-81.9%)→(1.3 mm diameter cold wiredrawn material: 62.1-71.8%) or→(1.3 mm diameter cold rolled material: 64.0-80.1%), but is considerably large in Comparative Examples 1 to 3 ((6 mm diameter hot rolled material: 80.1%)→(1.3 mm diameter cold wiredrawn material: 64.9%), (6 mm diameter hot rolled material: 83.0-85.9%)→(1.3 mm diameter cold rolled material: 62.5-64.4%).

The change of the above material characteristics is shown in FIGS. 10 to 12.

The above matter is further apparent by comparing the Examples and the Comparative Examples in each of those drawings. On the other hand, according to the recess shapability test of machine screw, there is the good case (Comparative Example 2) that recess crack does not generate in the case of previously subjecting the test material to the spheroidizing annealing treatment in Comparative Examples 2 and 3 even in the case that the tensile strength TS exceeds 1000 MPa (provided that crack generates in Comparative Example 3), but in the case of being in a cold rolled state without applying spheroidizing annealing, recess crack generates in both Comparative Examples 2 and 3. However, in Comparative Example 1 in which the tensile strength TS is less than 1000 MPa (962 MPa in 1.3 mm diameter of total reduction 95.3%), recess crack is good.

Thus, in the Comparative Examples that are outside the scope of the invention of this application, when the total reduction of cold wiredrawing or cold rolling to a raw material increases, thereby increasing the tensile strength to a certain value or more, crack generates when forming the recess of M1.6 pan-head machine screw that requires extremely severe cold headability, unless an appropriate softening treatment such as spheroidizing annealing is applied. Contrary to this, in the Examples, it is understood that even in a cold wiredrawn or cold rolled state, crack does not generate even in such a severe recess test unless the tensile strength TA exceeds 1500 MPa. Further, from the standpoint of the cold workability other than such a particularly severe cold headability, it is understood that even in the case that the level of the reduction of area RA takes as a measure, Examples 1 to 9 are superior to Comparative Examples 1 to 3. Next, considering the comparison between Examples 1 to 9 and Comparative Examples 1 to 3 from the difference in components of a steel material, it is understood that according to the process for producing a high-strength steel in accordance with the invention of this application, an excellent cold-working exhibiting steel wire that can maintain the high tensile strength TS in a high level range such as from 1000 to 1400 MPa, and further the reduction of area RA in a considerably high level of 65% or more can be obtained using an ultra-low carbon steel having C content of from 0.0014 to 0.0109 mass % as a raw material, in a cold worked state without conducting spheroidizing annealing (see FIG. 12).

The level of the tensile strength TS to C content in the steel wire in the case of 1.3 mm wire diameter is shown in FIG. 14, and a graph comparing the level of the reduction of area RA to C content in the steel wire in the same case of 1.3 mm wire diameter between Examples 1 to 9 and Comparative Examples 1 to 3 is shown in FIG. 1. The condition that the cold workability of 1.3 mm wire diameter is constant corresponds to the industrial strain of 3.06.

[V] (3) (b) Second Group of Comparative Example (Comparative Example 4)

As the second group of the Comparative Examples, a crude screw and a carburized and quenched screw produced from a steel wire corresponding to the commercially available SWCH16A produced by the conventional technology were used as Comparative Example 4.

This screw is M1.6 pan-head machine screw, and its chemical component composition is shown in component No. 9 in Table 27.

TABLE 27 JIS Chemical component Component corresponding composition (mass %) No. Supplied to component C Si Mn P S Sol. Al 9 Comparative SWCH16A 0.16 0.04 0.74 0.005 0.008 0.030 Example 4

Its production process is the conventional technology. There are two kinds of screws. One is that a steel wire rod is produced by hot rolling, the steel wire rod is subjected to cold wiredrawing by the conventional technology to produce a steel wire of 1.3 mm diameter, the steel wire is subjected to spheroidizing annealing to improve cold headability, and the steel wire is shaped into M1.6 pan-head machine screw by cold heading and rolling (a crude screw), and another is that the crude screw is subjected to carburizing quenching and tempering treatment to produce M1.6 pan-head machine screw having a predetermined strength imparted thereto (a carburized and quenched screw). As the characterization test of Comparative Example 4, the torsional torque test (same as described before) was conducted using the crude screw and the carburized and quenched screw as the test materials (referred to as “B2 group test materials”). The test results are shown in Table 28.

TABLE 28 JIS Vickers Torsional Test Component corresponding C content Kind of hardness breaking torque material Test No. to component (mass %) Screw Thermal refining Hv (—) (kgf × cm) C2 group Comparative 9 SWCH16A 0.16 M1.6 Crude — 1.82 Example 4 pan-head Carburizing 330 2.96 machine quenching and screw tempering

The following is understood from the above test results. In Comparative Example 4 produced by the production process that is outside the scope of the invention of this application, regarding the crude screw test material, the torsional breaking torque of M1.6 pan-head machine screw was the low value of 1.82 kgf×cm, but regarding the carburizing quenched screw, the high torsional strength of 2.96 kgf×cm is obtained, thus having the desired torsional strength. In the torsional torque test conducted in the Examples described before, the torsional strength was 2.63 kgf×cm in Example 6, but it exceeds 2.9 kgf×cm in all of the tests conducted in other Examples. Thus, it is understood to have the sufficient torsional strength. From the above tests, there were confirmed the industrial advantage of the excellent cold-workability exhibiting high-strength steel wire or steel bar and the high-strength shaped article according to the invention of this application, and the industrial advantage of the production process of the steel wire or steel bar and the high-strength shaped article according to the present invention as the production process for producing those. 

1-63. (canceled)
 64. An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less and being free of a cementite.
 65. An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ae₁ point, and having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less.
 66. An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a C content of 0.010 mass % or less, and having a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less.
 67. The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in any one of claims 64 to 66, having a tensile strength TS of 900 MPa or more.
 68. The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in any one of claims 64 to 66, having a reduction of area RA of 60% or more.
 69. The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in any one of claims 64 to 66, having a hardness of 285 or more in terms of Vickers hardness Hv.
 70. A high-strength shaped article, having a ferrite structure with an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less and being free of a cementite.
 71. A high-strength shaped article, having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ae₁ point, and having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less.
 72. A high-strength shaped article, having a C content of 0.010 mass % or less, and having a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less.
 73. The high-strength shaped article as claimed in any one of claims 70 to 72, having a tensile strength TS of 1000 MPa or more.
 74. The high-strength shaped article as claimed in any one of claims 70 to 72, having a hardness of 300 or more in terms of Vickers hardness Hv.
 75. An excellent cold-workability exhibiting high-strength steel wire or steel bar, having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in a cross section perpendicular to a longitudinal direction of the steel wire or steel bar of 200 nm or less.
 76. The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in claim 75, having a reduction of area RA of 65% or more.
 77. The excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in claim 75 or 76, having a tensile strength of 1000 MPa or more.
 78. A high-strength shaped article, having a C content of from more than 0.01 to 0.45 mass %, comprising, as a main phase, a ferrite structure having an average grain size in at least one cross section of cross sections in optional directions of 200 nm or less.
 79. The high-strength shaped article as claimed in claim 78, having a tensile strength TIS of 900 MPa or more.
 80. The high-strength shaped article as claimed in claim 78, having a hardness of 285 or more in terms of Vickers hardness Hv.
 81. A process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar, wherein a steel ingot, cast slab, steel slab or steel semifinished product having a cementite-free ferrite structure or having a C content of not greater than a solid solution limit of carbon in a ferrite phase at Ae₁ point or having a C content of 0.010 mass % or less is subjected to warm working to prepare a material having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 3 μm or less, and the material is then subjected to cold working to form a ferrite structure having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 200 nm or less.
 82. The process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in claim 81, wherein the warm working is a working such that at the working temperature in a range of from 350 to 800° C., a total reduction R represented by the following equation (1): R={(S ₀ −S)/S ₀}×100  (1) wherein R: total reduction (%) applied to cast slab or steel slab S₀: C directional cross section area of cast slab or steel slab just before initiation of warm working S: C directional cross section area of a material obtained after completion of warm working is 50% or more by rolling and/or forging and the cold working is a working such that at the working temperature of lower than 350° C., a total reduction R′ represented by the following equation (2): R′={(S ₀ ′−S′)/S ₀′}×100  (1) wherein R′: total reduction (%) applied to a warm rolled material S₀′: C directional cross section area of a material just before initiation of cold working S′: C directional cross section area of a material obtained after completion of cold working is 5% or more by rolling and/or drawing.
 83. A process for producing a high-strength shaped article, which comprises subjecting the excellent cold-workability exhibiting high-strength steel wire or steel bar produced by the above production process as claimed in any one of claim 81 or 82 to cold heading, cold forging and/or machining.
 84. A process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar, which comprises subjecting a steel ingot, cast slab, steel slab or steel semifinished product having a C content of from more than 0.01 to 0.45 mass % to warm working to prepare a material having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 3 μm or less, and then subjecting the material to cold working to form a ferrite main phase structure having an average crystal grain size in a cross section perpendicular to a longitudinal direction of 200 nm or less.
 85. The process for producing an excellent cold-workability exhibiting high-strength steel wire or steel bar as claimed in claim 84, wherein the warm working comprises working at a working temperature in a range of from 350 to 800° C., a total reduction R represented by the following equation (1): R={(S ₀ −S)/S ₀}×100  (1) wherein R: total reduction (%) applied to cast slab or steel slab S₀: C directional cross section area of cast slab or steel slab just before initiation of warm working S: C directional cross section area of a material obtained after completion of warm working is 50% or more is applied to the cast slab or the steel slab by rolling and/or forging and the cold working is a working wherein at the working temperature of lower than 350° C., a total reduction R′ represented by the following equation (2): R′={(S ₀ −S′)/S ₀′}×100  (1) wherein R′: total reduction (%) applied to a warm rolled material S₀′: C directional cross section area of a material just before initiation of cold working S′: C directional cross section area of a material obtained after completion of cold working is 5% or more is applied to the warm rolled material by rolling and/or drawing.
 86. A process for producing a high-strength shaped article, which comprises subjecting the excellent cold-workability exhibiting high-strength steel wire or steel bar produced by the above production process as claimed in claim 84 or 85 to cold heading, cold forging and/or machining. 